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Video (TG-16)
The TG-16 has two video control chips, a display controller and a color encoder.
Information on this page excerpted from http://archaicpixels.com/HuC6270 and http://archaicpixels.com/HuC6260.
Video Display Controller (HuC6270)
The video display controller (VDC) is the graphical workhorse of the TG-16.
Note: all VDC registers accept 16 bit values.
Specifications
- 64 simultaneous sprites
- 64KB of VRAM
- a background and foreground layer
Interface
[code]
Address | Access | Description
(Mapped | mode |
to $FF) | |
$0000 | R | 6270 Status register
| |
| | Different bits flag different conditions.
| | Not all are known.
| | (Note: can use special ST0 opcode to store
| | an immediate value.)
| | b 7 = 0
| | b 6 = 'BSY' flag
| | I believe this is '1' when a DMA transfer
| | is happening
| | b 5 = 'VD' flag
| | I believe this is a '1' when Vertical Sync
| | happens, otherwise a '0' (uncertain)
| | b 4 = 'DV' flag (unknown)
| | b 3 = 'DS' flag (unknown)
| | b 2 = 'RR' flag
| | Set during a Scanline interrupt (see RCR
| | register)otherwise '0'
| | b 1 = 'OR' flag (unknown)
| | b 0 = 'CR' flag (unknown)
| |
$0000 | W | 6270 Address register
| |
| | b 7-5 = ignored
| | b 4-0 = 6270 register number to access using
| | the 6270 data registers
| | ($0002 and $0003). Please see 6270
| | register list (SECTION 4) for details.
| |
$0002 | R/W | 6270 data LSB
| |
| | Note: can use special ST1 opcode to store
| | an immediate value.)
| |
$0003 | R/W | 6270 data MSB
| |
| | Note: can use special ST2 opcode to store
| | an immediate value.)
[/code]
Status Register
A bit corresponding to one of interruption jobs is set to be "H" in the status register to make the interruption active when a cause of the interruption which is enabled by an interruption permission bit of a control register and DMA control register as showing in Figures 3G and 3Q is occurred. When the status is read from the status register, the corresponding bit is cleared automatically.
The status indicating bits are as follows.
Read Behavior
| Bit(s) | Name | Description | Details |
|---|---|---|---|
| 0 | CR | sprite collision | Sprite #0 has collided with another sprite (1 to 63). |
| 1 | OR | sprite overflow |
|
| 2 | PR | scanline interrupt | A value of a raster counter becomes a predetermined value of a raster detecting register. |
| 3 | DS | VRAM to SATB end of transfer. | Data transfer between the VRAM and sprite attribute table buffer is finished. |
| 4 | DV | VRAM DMA end of transfer | Data transfer between two regions of VRAM has finished. |
| 5 | VD | vertical blanking | The VRAM accessed for the writing or reading of data by the CPU so that the BUSY terminals is "0". |
| 6 | BSY | DMA busy | A DMA transfer is in progress. |
| 7 - 15 | (unused) |
Write Behavior
| Bit(s) | Description |
|---|---|
| 0 - 4 | VDC register index (0-19) |
| 5 - 15 | (unused) |
Address Register
A register number "AR" is exclusive written into the address register designating one of the memory address write register to DMA VRAM-SATB source address register as shown in Figures 3C to 3U so that data are writing into the video display controller(1) under the condition that the A1 and CS terminals thereof are "L".
In a case where 16 bit data bus is selected, the EX 8/16 terminal is "0", the A1 terminal is "0", the "R/W" terminal is "W", and the A0 terminal is no matter.
In a case where 8 bit data bus is selected, the EX 8/16 terminal is "1", the A0 and A1 terminals are "0", and the "R/W" terminal is "W".
Data Register
Read/Write register.
Data in the VDC register selected/indexed by the Status Register.
Interface
[code]
Address | Access | Description
(Mapped | mode |
to $FF) | |
$0000 | R | 6270 Status register
| |
| | Different bits flag different conditions.
| | Not all are known.
| | (Note: can use special ST0 opcode to store
| | an immediate value.)
| | b 7 = 0
| | b 6 = 'BSY' flag
| | I believe this is '1' when a DMA transfer
| | is happening
| | b 5 = 'VD' flag
| | I believe this is a '1' when Vertical Sync
| | happens, otherwise a '0' (uncertain)
| | b 4 = 'DV' flag (unknown)
| | b 3 = 'DS' flag (unknown)
| | b 2 = 'RR' flag
| | Set during a Scanline interrupt (see RCR
| | register)otherwise '0'
| | b 1 = 'OR' flag (unknown)
| | b 0 = 'CR' flag (unknown)
| |
$0000 | W | 6270 Address register
| |
| | b 7-5 = ignored
| | b 4-0 = 6270 register number to access using
| | the 6270 data registers
| | ($0002 and $0003). Please see 6270
| | register list (SECTION 4) for details.
| |
$0002 | R/W | 6270 data LSB
| |
| | Note: can use special ST1 opcode to store
| | an immediate value.)
| |
$0003 | R/W | 6270 data MSB
| |
| | Note: can use special ST2 opcode to store
| | an immediate value.)
[/code]
Status Register
A bit corresponding to one of interruption jobs is set to be "H" in the status register to make the interruption active when a cause of the interruption which is enabled by an interruption permission bit of a control register and DMA control register as showing in Figures 3G and 3Q is occurred. When the status is read from the status register, the corresponding bit is cleared automatically.
The status indicating bits are as follows.
Read Behavior
| Bit(s) | Name | Description | Details |
|---|---|---|---|
| 0 | CR | sprite collision | Sprite #0 has collided with another sprite (1 to 63). |
| 1 | OR | sprite overflow |
|
| 2 | PR | scanline interrupt | A value of a raster counter becomes a predetermined value of a raster detecting register. |
| 3 | DS | VRAM to SATB end of transfer. | Data transfer between the VRAM and sprite attribute table buffer is finished. |
| 4 | DV | VRAM DMA end of transfer | Data transfer between two regions of VRAM has finished. |
| 5 | VD | vertical blanking | The VRAM accessed for the writing or reading of data by the CPU so that the BUSY terminals is "0". |
| 6 | BSY | DMA busy | A DMA transfer is in progress. |
| 7 - 15 | (unused) |
Write Behavior
| Bit(s) | Description |
|---|---|
| 0 - 4 | VDC register index (0-19) |
| 5 - 15 | (unused) |
Address Register
A register number "AR" is exclusive written into the address register designating one of the memory address write register to DMA VRAM-SATB source address register as shown in Figures 3C to 3U so that data are writing into the video display controller(1) under the condition that the A1 and CS terminals thereof are "L".
In a case where 16 bit data bus is selected, the EX 8/16 terminal is "0", the A1 terminal is "0", the "R/W" terminal is "W", and the A0 terminal is no matter.
In a case where 8 bit data bus is selected, the EX 8/16 terminal is "1", the A0 and A1 terminals are "0", and the "R/W" terminal is "W".
Data Register
Read/Write register.
Data in the VDC register selected/indexed by the Status Register.
VRAM Registers
$00 - MAWR - Memory Address Write Register (VRAM Write Address)
A starting address "MAWR" is written into the memory address write register so that the writing of data begins at the starting address of the VRAM(7).
MAWR specifies a word offset into VRAM for writing. Subsequent writes to register $02 (VWR) will store data at the offset specified by MAWR. After each write, MAWR is incremented by the amount specified in the IW bits of CR. MAWR wraps back to zero when it's value exceeds $FFFF.
The LSB and MSB of MAWR can be updated independently of each other; accessing either half directly updates the MAWR register rather than any temporary storage. This allows quick non-sequential addressing of VRAM without having to set the entire address every time.
$01 - MARR - Memory Address Read Register (VRAM Read Address)
A starting address "MARR" is written into the memory address read register. When the upper byte of the starting address is written thereinto, data are begun to be read from the starting address of the VRAM(7) so that data thus read are written into a VRAM data read register as showing in Figure 3F. There after, the starting address "MARR" is automatically incremented by one.
MARR specifies a word offset into VRAM for reading. When the MSB is written, VRAM data from the current offset is transferred into a read buffer, and then MARR is incremented by the amount specified in the IW bits of CR. For any following VRR reads, the buffered value is immediately returned and this process repeats; the buffer is loaded from data at the current offset and MARR is incremented again.
The LSB of MARR can be updated independently of the MSB. This does not cause the buffer to be loaded, only a write to the MSB will do that.
$02 - VRR - VRAM Data Read Register
A starting address "MARR" is written into the memory address read register. When the upper byte of the starting address is written thereinto, data are begun to be read from the starting address of the VRAM(7) so that data thus read are written into a VRAM data read register as showing in Figure 3F. There after, the starting address "MARR" is automatically incremented by one.
Reading the LSB of VRR returns the LSB of the read buffer. Reading the MSB returns the MSB of the read buffer immediately, then loads the buffer with VRAM from the current offset MARR represents and increments MARR by the value specified by the IW bits of CR. To read only the MSB of multiple words, the MSB of VRR can be repeatedly read instead of reading both the LSB and MSB.
Note: when reading from VDC addresses $0002 or $0003 when VRR is not selected, the buffer will not be reloaded nor will MARR increment when the MSB is read. The buffer contents will always return the last-loaded value but never update.
$02 - VWR - VRAM Data Write Register
Data which are transferred from the CPU(2) to the VRAM(7) are written into the VRAM data write register. When the upper byte of the data "VWR" is written thereinto, the video display controller(1) begins to write the data into the VRAM(7) and the address "MAWR" of the memory address write register is automatically incremented by one upon writing of the data.
When writing to VWR, the LSB is stored in a latch rather than VRAM. Any additional writes to the LSB only update the latch contents and do not affect VRAM. When the MSB is written to, the latched LSB and new MSB data are stored to VRAM at the current offset specified by MAWR. By loading the LSB with a given value and writing to the MSB repeatedly, you can fill VRAM with a constant LSB value and variable MSB value.
$05 - CR - Control Register
| Bit(s) | Description | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 - 3 | (IE) enable/[o]disable[/o] interrupt flags | ||||||||||||||||||||
| 0 | collision detection (between sprite #0 and any other sprites). | ||||||||||||||||||||
| 1 | sprite overflow, more than 16 sprites on a scanline. | ||||||||||||||||||||
| 2 | scanline match flag. | ||||||||||||||||||||
| 3 | vertical blanking. | ||||||||||||||||||||
| 4 | (EX) [o]input[/o]/output hsync signal | ||||||||||||||||||||
| 5 | (EX) [o]input[/o]/output vsync signal | ||||||||||||||||||||
| 6 | (SB) sprites enable/[o]disable[/o] flag | ||||||||||||||||||||
| 7 | (BB) background enable/[o]disable[/o] flag | ||||||||||||||||||||
| 8 - 9 | (DR) selects DISP terminal output (pin 27)
| ||||||||||||||||||||
| 10 | (DR) dynamic RAM refresh enable/[o]disable[/o] flag
Refresh address MA0-MA15 upon setting the flag in a case where a VRAM pixel width (see register $09) is of 2 pixels or 4 pixels in a Memory Width Register ($09) | ||||||||||||||||||||
| 11 - 12 | read/write address auto-increment
Affect by how much are incremented the address register $00 and $01. | ||||||||||||||||||||
| 13 - 15 | (unused) |
$06 - RCR - Raster Counter Register
A raster number "RCR" at which an interruption job is performed is written into the raster detecting register. An interruption signal is produced when a value of a raster counter is equal to the raster number "RCR". The raster counter is preset to be "64" at a preceding scanning raster line to a display starting raster line as described in more detail later, and is increased at each raster line by one.
| Bit(s) | Description |
|---|---|
| 0 - 9 | The rcr bit controls the generation of a raster counter IRQ. The VDC generates an IRQ, when the scanline specified in the RCR register is displayed. You need to add 64 to the RCR register to get the correct scanline. |
| 10 - 15 | (unused) |
A raster number "RCR" at which an interruption job is performed is written into the raster detecting register. An interruption signal is produced when a value of a raster counter is equal to the raster number "RCR". The raster counter is preset to be "64" at a preceding scanning raster line to a display starting raster line as described in more detail later, and is increased at each raster line by one.
$07 - BXR - Background X-Scroll Register
The BGX scroll register is used for a horizontal scroll of background on a screen. When
a content "BXR" is rewritten therein, the content is effective in the following raster
line.
The value written to BXR is latched on each scanline, preventing mid-scanline changes to BXR. Further changes to BXR will not change the display until the next scanline is displayed. When the VDC generates synchronization signals this duration is in units of VDC scanlines, and when the VDC inputs external synchronization signals this is in units of VCE scanlines.
For example if the VDC displays multiple VDC scanlines in one VCE scanline, the same BXR value applies to all VDC scanlines until the current VCE scanline ends.
$08 - BYR - Background Y-Scroll Register
The BGY scroll register is used for a vertical scroll of background on a screen. When a content "BYR" is rewritten therein, the content is effective to be as "BYR+1" in the following raster line.
The value written to BYR is latched on each scanline, preventing mid-scanline changes to BYR. Further changes to BYR will not change the display until the next scanline is displayed. When the VDC generates synchronization signals this duration is in units of VDC scanlines, and when the VDC inputs external synchronization signals this is in units of VCE scanlines.
For example if the VDC displays multiple VDC scanlines in one VCE scanline, the same BYR value applies to all VDC scanlines until the current VCE scanline ends.
$09 - MWR - Memory Width Register
Character cycles
The fundamental unit of time observed by the VDC is the duration of one pixel clock cycle. The pixel clock is output by the VCE and can be any of the following:
- 5.3693175 MHz - ~186 ns per pixel
- 7.15909 MHz - ~140 ns per pixel
- 10.738635 MHz - ~93 ns per pixel
The VDC accesses VRAM in groups called character cycles. Each character cycle can be split into eight slots, which have a duration of one pixel clock each. The actual VRAM read or write cycle spans one or more slots, selectable in units of 1, 2, or 4 slots each.
Here's a diagram showing the number of VRAM accesses that can be made in one character cycle depending on number of slots allocated to read or write cycle:
| VRAM cycle width | Slot 1 | Slot 2 | Slot 3 | Slot 4 | Slot 5 | Slot 6 | Slot 7 | Slot 8 |
|---|---|---|---|---|---|---|---|---|
| 1 slot | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 2 slots | 1 | 2 | 3 | 4 | ||||
| 4 slots | 1 | 2 | ||||||
Within the same period of time a character cycle spans, up to 8 accesses can be done when the VRAM access cycle width is 1 slot, 4 accesses can be done when cycle width is 2 slots, and only two access can be done when the cycle width is 4 slots.
The PCE uses 100ns SRAM chips as it's video RAM, so the only situation that is problematic is using a 1-cycle pixel width along with the 10.738635 MHz pixel clock. In this case each cycle is ~93 ns which violates the minimum access time requirements of the SRAM. In practice this does not cause any problems, however it does mean operating the memory 7% faster than it's guaranteed to work. This can be remedied by using a pixel width mode with longer cycles.
VRAM pixel width
The VDC will make as many sequential character cycles as the screen is wide as specified in the HDW field plus two, regardless of any horizontal scroll setting. These occur back-to-back in realtime as the display is rendered (I think there is a 1 or 2 character pipeline before any pixels are actually output). For example if the screen is 32 characters wide, 34 character cycles occur.
Bits 1, 0 of MWR set the VRAM access cycle grouping, referred to as the 'VRAM pixel width'. Bit 7 sets the character generator read mode when only two of four bitplanes can be read, due to insufficient VRAM access cycles available.
Bits 1-0 : VRAM pixel width.
| D1-D0 | Slot 1 | Slot 2 | Slot 3 | Slot 4 | Slot 5 | Slot 6 | Slot 7 | Slot 8 |
|---|---|---|---|---|---|---|---|---|
| 00 | CPU | BAT | CPU | CPU | CG0 | CPU | CG1 | |
| 01 | BAT | CPU | CG0 | CG1 | ||||
| 10 | BAT | CPU | CG0 | CG1 | ||||
| 11 | BAT | CG0 / CG1 | ||||||
- BAT is a read from the BAT region of VRAM. (word contains palette, character name)
- CPU is a CPU access, either read or write.
- CG0 is a read from the character generator region of VRAM. (word contains bitplane 0 and 1 bytes)
- CG1 is a read from the character generator region of VRAM. (word contains bitplane 2 and 3 bytes)
The first three modes function identically. The last mode only has enough spare time in each character cycle to read CG0 or CG1, but not both. Selection of either bitplane group is done by the character generator mode bit (CM), which is bit 7 of MWR. It specifies 0= CG0 or 1= CG1. Internally, the VDC assumes the missing bitplane data is forced to zero. This means that tiles displayed when CM=0 use colors 0,1,2,3, and tiles displayed when CM=1 use colors 0,4,8,C.
Sprite pixel width
During the horizontal blanking period, the VDC fetches character generator data for the sprites (up to 16) that passed y-evaluation and have their respective data buffered in the VDC's on-chip sprite storage. The bitplane data is loaded into shift registers and will be output serially during the next scanline.
The duration of the fetch period directly relates to how much horizontal blanking time is available, as defined by the HSW, HDS, HDW, and HDE registers. If the period is too short, the process is aborted. It seems the sprites that weren't loaded have their shift registers reset to zero, as previously loaded sprite data or garbage data is not shown (this needs more testing).
Much like background rendering, bits 3-2 of MWR set the character cycle allocation for sprites, referred to as the 'Sprite pixel width'.
Bits 3-2 : Sprite pixel width.
| D1-D0 | Slot 1 | Slot 2 | Slot 3 | Slot 4 | Slot 5 | Slot 6 | Slot 7 | Slot 8 |
|---|---|---|---|---|---|---|---|---|
| 00 | SP0 | SP1 | SP2 | SP3 | SP0 | SP1 | SP2 | SP3 |
| 01 | SP0, SP2 | SP1, SP3 | SP0, SP2 | SP1, SP3 | ||||
| 10 | SP0 | SP1 | SP2 | SP3 | ||||
| 11 | SP0 / SP2 | SP1 / SP3 | ||||||
- SP0-3 are sprite bitplanes 0,1,2,3.
- 00b reads data for two sprites in one cycle.
- 01b reads data for two sprites in two cycles (bitplanes 0,1 for sprites 1,2 in cycle, bitplanes 2,3 for sprites 1,2 in the next).
- 10b reads data for one sprite in one cycle.
- 11b reads data for one sprite in one cycle, but only bitplanes 0,1 or 2,3 can be read.
Bit 0 of the pattern code field of each sprite entry specifies which bitplanes are read for a sprite pixel width setting of 11b. It can be 0= SP0,SP1 or 1= SP2,SP3. The unused bitplanes are forced to zero so that the colors used out of a 16-color palette are 0,1,2,3 when SP0,SP1 are read, or 0,4,8,C when SP2,SP3 are read.
Display Registers
$0A - HPR - Horizontal Synchronous Register
Bits 0-4 : Horizontal Sync Width (HSW)
Bits 8-14 : Horizontal Display Start (HDS)
HSW defines the width of the horizontal sync pulse in 8-pixel (character) units. The range is 1 to 32 characters.
HDS defines the interval after the horizontal sync pulse to the start of the horizontal display period in character units. The range is 1 to 128 characters.
When the VDC inputs external synchronization signals, the function of HSW changes. It no longer affects the width of the horizontal sync pulse. Instead, if during the processing of any VDC-generated scanline the HDE state expires prior to an external [o]HSYNC[/o] pulse, the number of characters as specified by HSW are taken up before the next VDC-generated scanline starts.
This distinction is important; increasing values of HSW do not displace the horizontal display area immediately following an external [o]HSYNC[/o] pulse, but they will for all subsequent VDC-generated scanlines before /HSYNC occurs again.
$0B - HDR - Horizontal Display Register
Bits 6-0 : Horizontal Display Width (HDW)
Bits 14-8 : Horizontal Display End (HDE)
HDW defines the width of the horizontal active display period in character units. The range is 1 to 128 characters.
HDE defines the interval following HDE to the end of the scanline, at which poinst the HSW state is entered and a horizontal sync pulse is generated. The range is 1 to 128 characters. It should be set to the remainder from the desired number of characters per scanline, minus HSW, HDS, and HDW.
$0C - VSR - Vertical Synchronous Register
Bits 0 to 4 (VSW) - vertical synchronous pulse width
A pulse width of a vertical synchronous signal is decided in a width of "L" level as a unit of a raster line. One of 1 to 32 is selected to comply with a specification of a CRT display.
Bits 8 to 15 (VDS) - vertical display starting position
A period between a rising edge of a vertical synchronous signal and a vertical synchronous starting position is set as an unit of a raster line. When it is assumed that a vertical display starting position (vertical back porch) is "N", "N-2" is written into the bits.
$0D - VDR - Vertical Display Register
A vertical display period (display region) is set as an unit of a raster line. A vertical display width is decided in accordance with the number of raster lines to be displayed on a CRT display which is defined by a content of the 9 bits. When it is assumed that a vertical display width is "N", "N-1" is written into the VDW bits.
$0E - VCR - Vertical Display Ending Postition Register
A period between a vertical display ending position and a rising edge of a vertical synchronous signal is set as an unit of a raster line. When it is assumed that a vertical optimum position (vertical front porch) is "N" to be defined by the 8 bits, "N" is written into the VCR bits.
Bits 7-0 : Vertical Display Position End (VCR)
VCR defines the interval following VDW to the end of the frame, at which point the VSW state is entered and a vertical sync pulse is generated. The range is 0 to 255 scanlines. It should be set to the remainder from the desired number of scanlines per frame, minus VSW, VDS, and VDW.
When the VDC inputs external synchronization signals, VCR should be set to a value equal to or larger than the number of scanlines the hardware generates from one edge of /VSYNC to the next. Otherwise the VDC will start generating another frame within the current display frame. This can be used to arbitrarily force additional VD interrupts and VRAM to SAT DMA transfers within a single VCE-defined frame.
DMA Registers
$0F - DCR - DMA Control Register
(1) bit 0 (DSC) - enable and interruption at the finishing of transfer between the
VRAM(7) and sprite attribute table buffer (23)
It is decided whether or not an interruption is enabled at the finishing time of
the transfer.
(1.1) "0" - disable
(1.2) "1" - enabled
(2) bit 1 (DVC) - enable of interruption at the finishing of transfer between two
regions of the VRAM(7).
It is decided whether or not an interruption is enabled finishing time of the
transfer
(2.1) "0" - disable
(2.2) "1" - enabled
(3) bit 2 (SI/D) - increment/decrement of a source address
One of automatical increment and decrement of a source address is selected in a
transfer between two regions of VRAM(7).
(4.1) "0" - disable
(4.2) "1" - enabled
(5) bit 5 (DSR) - repetition of a transfer between the VRAM(7) and the sprite
attribute table buffer(23) is enabled.
$10 - SOUR - Source Address Register
A starting address of a source address is allocated in a transfer between two regions
of the VRAM(7).
$11 - DESR - DMA Destination Address Register
A starting address of a destination address is allocated in a transfer between two
regions of the VRAM(7).
$12 - LENR - DMA Block Length Register
A length of a block is defined in a transfer between two regions of the VRAM(7).
$13 - SATB - Sprite Attribute Table Address Register
The address of the Sprite Attribute Table. This is the only address used for access to the SATB.
VRAM Access
Typically when loading large amounts of data into VRAM the screen is turned off for several frames. In most video hardware turning the screen off stops display related DMA and gives the CPU full access to VRAM.
The VDC handles things a bit differently. BURST mode is when the color bus outputs $0100 on VD8-VD0 (sprite palette #0, color #0), display DMA is stopped (no fetching of BAT data, background patterns, sprite patterns), and the CPU has unrestricted access to VRAM regardless of the MWR settings. BURST mode is enabled in two situations:
1. Any display state outside of VDW is considered to be in the BURST mode. A possible exception is that display DMA needs to be done on line 262 or 263 (depending on the frame height) for graphics to be displayed on scanline 0.
2. If bits 7 and 6 of CR are reset prior to VDW occurring, BURST mode is forcibly entered for the entire duration of VDW. Any changes to bits 7 and 6 have no effect until the next transition into VDW, at which point they are sampled again.
Note that this means simply turning off the background and/or sprites during VDW does *not* select BURST mode, and VRAM access is still restricted. When the background is turned off during the display, the color bus outputs $0000 on VD8-VD0 (background palette #0, color #0).
To maximize VRAM throughput, it isn't necessary to force a BURST-in-VDW display condition. The duration of VDW can just be shortened to letterbox the screen and allocate more scanlines to to the other display periods, giving more BURST time.
A MWR setting of $00 gives the CPU the largest amount of access cycles (twice per 8 pixels) which seems to be exactly equal to the amount of accesses available during BURST mode.
|}
Research Notes
The following is a copy of the original research notes put together by the TG-16/PC Engine hacking community regarding the VDC. If you were confused by the above, the more human and direct tone of these notes may be more comfortable.
[code]
***************************************************************** * PC-Engine Video Display Controller Documentation * * . . * * ---+----------------------------------------------+--- * * | MOST COMPLETE HuC6270 INTERNAL WORKINGS | * * | DOCUMENT. IF YOU HAPPEN TO FIND *ANY* | * * | WRONG INFORMATION, PLEASE CONTACT ME VIA | * * | EMAIL AS SOON AS POSSIBLE SO I CAN FIX IT. | * * ---+----------------------------------------------+--- * * : : * * * * document revision 0.3 (3rd release) * * * * written by Emanuel Schleussinger in Feb 1998 * * ( eschleus@luva.lb.bawue.de ) * * Thanks to: * * DAVID MICHEL for LOTS of information!!!!!!!!!! ;) * * JENS CHR. RESTEMEIER for his EXCELLENT PCE-docu * * DAVE SHADOFF for his emails and his TGSim source * * NIMAI MALLE for his VDC explanations * * VIDEOMAN for his excellent Hacking Web-Page * * and some documents in there * * PAUL CLIFFORD for an excellent HuC6270 register docu * *****************************************************************
Revision reference: -----+-----------------------------+---------------+---------- | rev 0.3: | - improved the VDC register table A LOT thanks to the | help of PAUL CLIFFORD. Thx for that cool doc, dude! | (all those nasty 'unknown's are now eliminated) | - more examples here and there. | - fixed some docu bugs with help of David Michel. | - Added Video Color Encoder reference. | - Sprite storage description was WRONG, corrected now. | | rev 0.2: | - Added Sprite information. | - Fixed some major bugs in the docu. | - Registers updated. | | rev 0.1 (initial release): | - Still missing sprite docu, lots of undocumented registers. -----+-----------------------------+---------------+----------
Document preface:
This document has been created for both beginners and advanced programmers. There may be some information that you may well consider 'unnecessary' (such as the introduction to planar image storage), but please think of people who would really like to program the PC-Engine, but dont have a clue on how some basic techniques (like planar) work.
This document is in very early state and may well contain a lot of information not being correct. For any wrong in- formation in this document you may discover, please write me a mail at eschleus@luva.lb.bawue.de so I can fix it and release a new version. The latest version of this document can always be obtained at my homepage located at:
www.classicgaming.com/aec/
or just write me an email and ask me to send you the latest revision.
Any help on improving this document is highly appreciated!
I think its the most complete one out there at this time.
Yours, Manuel
eschleus@luva.lb.bawue.de www.classicgaming.com/aec/
-------------------------------------------------------- ----- T O P I C S ------------- --------------------------------------------------------
+-------------------------------------------+
| 1. Purpose of the VDC / General info |
| |
| 2. The VRAM structure / encoding VRAM data|
| |
| 3. Accessing the VDC from the CPU |
| |
| 4. The VDC registers in detail |
| |
| 5. The Sprites in the VRAM |
| |
| 6. The Sprite attribute table (SATB) |
| |
| 7. The Video Color Encoder |
| |
+-------------------------------------------+
-------------------------------------------------------- ----- 1. Purpose of the VDC / General info ------------- --------------------------------------------------------
The VDC (Video Display Processor), also known as the HuC6270, is the main graphics processing unit in the PC-Engine. Despite the CPU of the PC-Engine is only 8-bit, the VDC is a full 16-bit processor with very powerful capabilities. Its accessible from the main system via 3 special opcodes that write/read data from/ into the Video Display. The VDC is connected to another chip known as the HuC6260 VCE (Video Color Encoder), which supplies the color palette data for the Video System.
The VDC in the PC-Engine has two modes of operation:
1. Background character processing 2. Sprite processing
The 64 kB VRAM that the VDC is connected to does NOT contain one big bitmap with all the display information stored pixel by pixel like on a Amiga or PC, the Graphics are stored tile-based. In case you do not know what tile-based graphics are, be sure to read section 2 very carefully.
-------------------------------------------------------- ----- 2. The VRAM structure / encoding VRAM data ------- --------------------------------------------------------
The VRAM of the PC-Engine is 64 kBytes in size. No chip other than the VDC can access it. It contains all the important data needed for the display generation.
The way graphical data is organized in the VRAM is called 'tile based'. This means there is NOT a huge bitmap containing a color index for every pixel, but only a list of pointers to small, rectangular areas in the VRAM that will, aligned to each other, make up the display. Explanation follows.
Think of it like this: We have a 512*256 pixel 256 color screen. On a PC, for instance, we would have to have the following VRAM structure:
+---------------------------------------+
| <--512 pixels across --> |
| |
| | 256
| | pixels
| | down
| |
| |
| |
| |
| |
+---------------------------------------+
The color depth is 8 bit ^= 256 colors
This would result in 512 * 256 * 8
= 1.048.576 bit
= 130 kbytes (roughly)
So, if the PC-Engine would do it the same way, it would not be able to have such high resolutions due to the lack of VRAM. Thats why data is stored in the VRAM as follows:
The screen background area is made up out of 8*8 pixels large blocks, called the 'tiles', each tile having a color palette of 16 colors. There are 16 different palettes to choose from for each tile, resulting in 256 different colors for the background generation (the other 256 colors are reserved for sprite usage which will be described later). In the background, colour 0 of all palettes are equal. Colour 0 of palette 0 determines colour 0 of all the background palettes. Even though these colour CAN be set independently, the screen will not reflect these settings.
Now, how are those tiles aligned to each other?
Starting at the very beginning of the VRAM ($0) there is the so- called BAT (Block Attribute Table), which is a list of pointers to tiles stored in the Video RAM. The amount of pointers varies depending on how big the actual screen is. (As I told you, you have 8*8 pixel tiles, so if the screen is larger, theres more tiles). For our test screen (512*256), we would need:
512 / 8 = 64 tiles per line
256 / 8 = 32 tiles vertical
64 * 32 = 2048 tiles
That means, we would be in need of a BAT 2048 words in lenght.
WHY WORDS? How does a BAT pointer to the VRAM look like?
A Pointer to a tile in the VRAM must contain palette information as well as the actual VRAM address where to find the tile. This ONE WORD LONG index pointer looks like this:
PPPPAAAAAAAAAAAA
| |
| |
| +------- 12 lower bits: Index of the tile
|
+--------------- 4 upper bits: Palette number (0-15)
If you multiply the tile index by 32 (LSL #5 ;-), you will get the actual VRAM pointer address.
The pointers in the BAT are ordered from the left to the right and top to down.
----->Small example:<-----
Here is the first few words of data in the VRAM of HATRIS, just having the intro screen up. If you look closely, note how VRAM was saved using the same tiles over and over again in the BAT:
HOW CAN I SET THE SIZE OF THE BAT?
Easy, theres a VDC register dedicated to it, called the MWR register. (find more about the MWR in SECTION 4)
MWR register mask:
xxxxxxxxxxHWWxxx (16 bits)
| |
| +--- width in tiles/pixels
| 00 = 32/256
| 01 = 64/512
| 10 = 128/1024
| 11 = 128/1024
|
+----- height in tiles/pixels
0 = 32/256
1 = 64/512
If you understood everything, you should now be asking: "No TV can display a resolution of 1024 pixels across, so whats this mode for?"
Answer: Check out the BXR and BYR registers used for SCROLLING ;)
(see SECTION 4)
HOW DOES THE TILE ITSELF LOOK LIKE IN THE VRAM?
Well, the tile itself is a piece of memory sized like this:
8 * 8 * 4 bits = 256 bits
| | |
| | +------- color index (4 bits per pixel)
| +----------- height in pixels
+--------------- width in pixels
On this issue, David Michel posted me a VERY good explanation on how the data of a single tile is organized in the VRAM:
The PC-Engine use a planar mode rather than the well known chunky mode of PCs, if you already have some experience decoding Atari ST or Amiga gfx, you should easily understand the following.
In planar mode the 4 bits that form the color index are stored in 4 separate bytes, let's say that we want to extract the color index for the third pixel from the left :
color index
3rd pixel
+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 3 | 2 | 1 | 0 | byte 1 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+---+---+---+---+ +---+---+---+---+---+---+---+---+
| | | | |
| | | +-----------------------+
| | |
| | | +---+---+---+---+---+---+---+---+
| | | byte 2 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| | | +---+---+---+---+---+---+---+---+
| | | |
| | +---------------------------+
| |
| | +---+---+---+---+---+---+---+---+
| | byte 3 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| | +---+---+---+---+---+---+---+---+
| | |
| +-------------------------------+
|
| +---+---+---+---+---+---+---+---+
| byte 4 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| +---+---+---+---+---+---+---+---+
| |
+-----------------------------------+
It's as simple as that :)
The funny part is that those 4 bytes are not placed in order, they are interleaved. Byte 1 & 2 are stored first, and bytes 2 & 3 are stored 16 bytes after, here is another nice drawing:
VRAM OFFSET
on pointer
(in bytes)
+---------------------+
0 | byte 1 & 2 of line 1|
+---------------------+
2 | byte 1 & 2 of line 2|
+---------------------+
4 | byte 1 & 2 of line 3|
+---------------------+
6 | byte 1 & 2 of line 4|
+---------------------+
8 | byte 1 & 2 of line 5|
+---------------------+
10 | byte 1 & 2 of line 6|
+---------------------+
12 | byte 1 & 2 of line 7|
+---------------------+
14 | byte 1 & 2 of line 8|
+---------------------+
+---------------------+
16 | byte 3 & 4 of line 1|
+---------------------+
18 | byte 3 & 4 of line 2|
+---------------------+
20 | byte 3 & 4 of line 3|
+---------------------+
22 | byte 3 & 4 of line 4|
+---------------------+
24 | byte 3 & 4 of line 5|
+---------------------+
26 | byte 3 & 4 of line 6|
+---------------------+
28 | byte 3 & 4 of line 7|
+---------------------+
30 | byte 3 & 4 of line 8|
+---------------------+
I think everyone should have got that right now. Thx David! If you ask yourself what this was about, consider reading part 2 again. Part 3 won't be better ;-)
-------------------------------------------------------- ----- 3. Accessing the VDC from the CPU ---------------- --------------------------------------------------------
---HOW CAN I TRANSFER DATA INTO THE VRAM?
Well, there are three memory locations involved that can be read/ written by the CPU to supply the VDC with data / read data from the VDC (all in the I/O Memory Segment $FF):
Full address Access Purpose
$1FE000 R/W VDC Register select $1FE002 R/W Low Data register $1FE003 R/W High Data register
The first of the three locations here is the so-called REGISTER SELECT. The VDC has 19 Registers (several of them being totally unknown, btw) to access. To tell the VDC to which register you want to write the value contained in $1FE002 (and $1FE003), simply write the number of the register to write to into the low 5 bits of $1FE000. As the VDC is a 16 bit processor (ALL VDC registers are one word wide) in most cases you will need to supply both of the data values.
Detailed description of the VDC ports by Videoman (slightly changed):
Address | Access | Description
(Mapped | mode |
to $FF) | |
+--------+---------------------------------------------------
$0000 | R | 6270 Status register
| |
| | Different bits flag different conditions.
| | Not all are known.
| | (Note: can use special ST0 opcode to store
| | an immediate value.)
| | b 7 = 0
| | b 6 = 'BSY' flag
| | I believe this is '1' when a DMA transfer
| | is happening
| | b 5 = 'VD' flag
| | I believe this is a '1' when Vertical Sync
| | happens, otherwise a '0' (uncertain)
| | b 4 = 'DV' flag (unknown)
| | b 3 = 'DS' flag (unknown)
| | b 2 = 'RR' flag
| | Set during a Scanline interrupt (see RCR
| | register)otherwise '0'
| | b 1 = 'OR' flag (unknown)
| | b 0 = 'CR' flag (unknown)
| |
$0000 | W | 6270 Address register
| |
| | b 7-5 = ignored
| | b 4-0 = 6270 register number to access using
| | the 6270 data registers
| | ($0002 and $0003). Please see 6270
| | register list (SECTION 4) for details.
| |
$0002 | R/W | 6270 data LSB
| |
| | Note: can use special ST1 opcode to store
| | an immediate value.)
| |
$0003 | R/W | 6270 data MSB
| |
| | Note: can use special ST2 opcode to store
| | an immediate value.)
+--------+---------------------------------------------------
----->One short example on this one:<------
To read the contents of Register 2 (VRAM-Read-Register) simply use the following line of code:
ST0 #2
...and then the two data values will sort of 'mirror' the value in
this VDC register.
-------------------------------------------------------- ----- 4. The VDC registers ----------------------------- --------------------------------------------------------
This huge and very complete list has been taken from Videomans hardware map document, Jens' PCE documentation, and some information to it was added by me.
REG ACCESS DESCRIPTION+
NO. MODE DETAILS
0 R?/W MAWR - 'Memory Address Write Register'
b 15-0 this is the internal
register used as an address-counter when writing to VRAM.
All bits used (although no VRAM above $7FFF).
1 R?/W MARR - 'Memory Address Read Register'
b 15-0 this is the internal
register used as an address-counter when reading from VRAM.
All bits used (although no VRAM above $7FFF).
2 R VRR - 'VRAM Read Register'
b 15-0 this is the only valid read-access
from the data port. It reads the value from VRAM at the
address specified by the MARR. When the value is read
from the second byte-port at $0003, the MARR register
(ie. the 'address to read from') is auto-incremented
(although this may be a configurable behaviour).
All bits used.
2 W VWR - 'VRAM Write Register'
(write-access version of the above)
b 15-0 Write value to VRAM at the address specified by the MAWR.
When the value is written to the second byte-port at $0003,
the MAWR register (ie. the 'address to write to') is
auto-incremented (although this may be a configurable behaviour).
3 ? (unused) ?
4 ? (unused) ?
5 ? CR - 'Control Register'
b 15-13 unused
b 12-11 'IW' Address register auto-Increment
of the MAWR register
00 - normal increment (+1)
01 - +32
10 - +64
11 - +128
b 10 'DR' Dynamic RAM refresh (unknown by me though)
b 9-8 'TE' Selection of DISP terminal outputs
00 - DISP output "H" during display
01 - BURST colour burst inserting position is
indicated by output "L"
10 - INTHSYNC internal horizontal synchronous
signal
11 - not used
b 7 'BB' background (on/off) --+
1 - display background |
0 - no background > gets effective in
b 6 'SB' sprites (on/off) | next horizontal
1 - display sprites | display period.
0 - no sprites --+
b 5-4 'EX' (name unknown by me)
00 - vsync and hsync inputs
01 - vsync input, hsync output
10 - not used
11 - vsync and hsync outputs
b 3 irq (on/off)
0 = disabled
1 = enabled
b 2 rcr (on/off)
0 = disabled
1 = enabled
b 1 Enable interrupt for excess number detection of
sprites.
0 = disabled
1 = enabled
b 0 Enable interrupt for sprite collision detection.
0 = disabled
1 = enabled
Editor's note: bits 3-0 sound suspiciously like
interrupt-enable flags. Given what we know about
the interrupt vector table, is it logical to assume
that the remaining two IE bits stand for the
remaining two interrupt vectors? Then again, maybe not.
$FFFC-$FFFD NMI Vector
$FFFA-$FFFB TIMER Vector
$FFF8-$FFF9 IRQ1 Vector (for Video)
$FFF6-$FFF7 IRQ2 Vector (for BRK)
6 R RCR - 'Raster Counter Register'
b 15-10 ?
b 9-0 The rcr bit controls the generation of a raster
counter IRQ. The VDC generates an IRQ, when the
scanline specified in the RCR register is displayed.
You need to add 64 to the RCR register to get the
correct scanline.
7 R?/W BXR - 'Background X-Scroll Register'
b 15-10 (not used)
b 9-0 when the background map is a larger virtual
size than the viewing screen shows, this is
the viewing screen's x-offset (in pixels)
from the origin of the virtual background map.
8 R?/W BYR - 'Background Y-Scroll Register'
b 15-9 (not used)
b 8-0 when the background map is a larger virtual
size than the viewing screen shows, this is
the viewing screen's y-offset (in pixels)
from the origin of the virtual background map.
9 R?/W MWR - 'Memory-access Width Register'
Used to configure the size of the virtual background
map.
b 15-8 (not used)
b 7 'CM' (unknown - presumably 'Color Mode')
b 6-4 'SCREEN' These bits control virtual map size
as noted below.
b 6 virtual screen height
0 = 256 pixels / 32 tiles
1 = 512 pixels / 64 tiles
b 5-4 virtual screen width
00 = 256 pixels / 32 tiles
01 = 512 pixels / 64 tiles
10 = 1024 pixels / 128 tiles
11 = 1024 pixels / 128 tiles
Complete lookup of
available sizes in tiles:
-------------------------
000 - 32 x 32
001 - 64 x 32
010 - 128 x 32
011 - 128 x 32
100 - 32 x 64
101 - 64 x 64
111 - 128 x 64
b 3-2 Sprite pixel width
b 1-0 VRAM pixel width
10($A) ? HSR - 'Horizontal Sync Register'
b 15 (not used)
b 14-8 'HDS' Horizontal display start position -1.
b 7-5 (not used)
b 4-0 'HSW' Horizontal synchronous pulse width.
Mask = $7F1F
11($B) ? HDR - 'Horizontal Display Register'
b 15 (not used)
b 14-8 'HDE' Horizontal display ending period -1.
b 7 (not used)
b 6-0 'HDW' Horizontal display width in tiles -1.
Mask = $7F7F
added from Jens' PCE-documentation: Lower half of HDR:
It controls the horizontal width of display generation.
The value in this register is the number of horizontal
tiles minus one. Normal values are 31, for 32 tiles
and 256 pixel horizontally, 39, for 40 tiles or 320
pixel and 63, for 64 tiles or 512 pixel.
12($C) ? VPR - 'Vertical synchronous register'
b 15-8 'VDS' Vertical display start position -2.
b 7-5 (not used)
b 4-0 'VSW' Vertical synchronous pulse width.
Mask = $FF1F
13($D) ? VDW - 'Vertical display register'
b 15-9 (not used)
b 8-0 Vertical display width in pixels -1.
NOTE:
Unlike the HDR register, the information on the
vertical display width is split up in two registers,
this one storing the vertical width, and the next one
(VCR) containing the vertical display end position.
14($E) ? VCR - 'Vertical display END position register'
b 15-8 (not used)
b 7-0 Vertical display end position.
15($F) ? DCR - 'DMA Control Register'
The DCR, SOUR, DESR and LENR registers control DMA operations. The DMA operation starts, as soon as the length is written into the LENR register
b 15-5 (not used)
b 4 - DSR DMA (VRAM-SATB transfer repetition)
b 3 - Increment (0)/decrement (1) of
destination address.
b 2 - Increment (0)/decrement (1) of
source address.
b 1 - Enable interrupt at completion of
VRAM-VRAM transfer.
Checked on completion of transfer.
b 0 - Enable interrupt at completion of
VRAM-SATB transfer.
Checked on completion of transfer.
16($10) R?/W SOUR - '(DMA) Source Address Register'
b 15-0 This register sets the source address
for DMA transfers.
All bits used (address pointer).
17($11) R?/W DESR - '(DMA) Destination Address Register'
b 15-0 This register sets the destination
address for DMA transfers.
All bits used
(although no VRAM above $7FFF).
18($12) R?/W LENR - '(DMA) Block Length Register'
b 15-0 This register sets the length of
the DMA transfer.
All bits used
(although no VRAM above $7FFF).
19($13) R?/W SATB - 'Sprite Attribute Table'
b 15-0 This register points to the start address
of the sprite attribute table.
All bits used
(although no VRAM above $7FFF).
-------------------------------------------------------- ----- 5. The Sprites in the VRAM ----------------------- --------------------------------------------------------
Well, I will not try to explain what Sprites are here. Basically, all of the PC-Engines' sprites are 16*16 to 32*64 pixels in size, and have a sprite palette of 16 colors. There are 16 separate sprite palettes available. (remember, there was 16*16 colors for the background processing, those colors are INDEPENDENT from the sprite palettes).
In the Sprites colours, colour 0 is transparent in all palettes, although it does peek it's head in a peculiar place; beyond the display width of the BG. Explanation: The background display area (in it's most often used setting) is 256x216. The display width of a television may be adjusted to squash the screen vertically, or horizontally. Even normal TVs show a little more that 256 TG-16 pixel wide, leaving a black border on the sides. This border colour is actually controlled by sprite colour 0. The programmer can actually set the screen width more narrow or vertically shorter, showing more of this area. It's only use that I've ever implemented was in measuring the CPU load of the TG-16 during development.
--HOW ARE SPRITES STORED IN THE VRAM?
For the sprite characters the principe is the same as for the background tiles, but in place of using bytes (8 pixels) they use words (16 pixels). Note that the words still use the same encoding as all word data within the PC-Engine, this means that the first byte of the word is the lower byte. Sprite data is stored like in the following drawing:
Byte Data
offset
+-------------------+
0 | plane 1 of line 1 |
+-------------------+
2 | plane 1 of line 2 |
+-------------------+
. .
: :
30 | plane 1 of line 16|
+-------------------|
32 | plane 2 of line 1 |
+-------------------+
34 | plane 2 of line 2 |
+-------------------+
36 | plane 2 of line 3 |
+-------------------+
. .
: :
46 | plane 2 of line 16|
+-------------------+
48 | plane 3 of line 1 |
......and so on.
Not only you can display sprites, you can do some sort of funny stuff with them, like mirroring, for instance. All this is controlled in the so-called SPRITE ATTRIBUTE TABLE.
-------------------------------------------------------- ----- 6. The Sprite attribute table (SATB) ------------- --------------------------------------------------------
The sprites' positions and attributes are defined in the so- called SPRITE ATTRIBUTE TABLE (SATB). The SATB can be contained any- where in the VRAM ($0000-$7FFF).
--HOW DOES THE VDC KNOW WHERE THE SATB IS TO BE FOUND?
The VDC has a special register containing nothing but the start address of the SATB in the VRAM. This is register 19 (SEE SECTION 4)
The actual sprite attributes are stored at the address mentioned above. For aech sprite, there is a 4 word long attribute section, which looks as follows:
Word | Access | Description offset | mode |
0 R/W Y position
b 15-10 (unused)
b 9-0 y position (relative to
virtual-screen origin)
1 R/W X position
b 15-19 (unused?)
b 9-0 x position (relative to
virtual-screen origin)
2 R/W Pattern address
b 15-11 (unused?)
b 10-0 sprite data VRAM address shifted
right 5 bits(Shift left 6 bits to
get real VRAM address)
3 R/W Sprite attributes
b 15 y-invert flag (upside-down)
b 14 unused
b 13-12 'CGY'
00 = sprite is 1 'cell' (16 pixels) high
01 = sprite is 2 cells high (32 pixels)
10 = invalid
11 = sprite is 4 cells high (64 pixels)
b 11 x-invert flag (left-right invert)
b 10-9 unused
b 8 'CGX'
0 = sprite is 1 'cell' wide (16 pixels)
1 = sprite is 2 cells wide (32 pixels)
b 7 'SPBG'; is sprite in foreground (in front
of CG) or background (behind CG)
b 6-4 unused
b 3-0 sprite colour (i.e. which of 16 sprite
palettes to use)
[/code]
Video Color Encoder (HuC6260)
The VCE has two functions:
- supply the picture on your television.
- define the location of the palettes in memory.
All VCE registers are 16 bit.
$0400 - CR - Control Register
Write only register.
| Bit(s) | Description | Values |
|---|---|---|
| 0 - 1 | PCC - Pixel Clock Control |
00 = 5.3693175 MHz 01 = 7.15909 MHz 10 = 10.738635 MHz 11 = 10.738635 MHz |
| 2 | Frame/Field Configuration |
0 = 262-line frame 1 = 263-line frame |
| 3 - 6 | ??? | ??? |
| 7 | Strip Colorburst |
0 = Colorburst intact 1 = Strip colorburst |
| 8 - 15 | (unused) |
$0402 - CTA - Color Table Address Register
Write only register.
| Bit(s) | Description | Values |
|---|---|---|
| 0 - 8 | index for the color table | 0 to 511 |
| 9 - 15 | (unused) |
Note: This register is auto-incremented after each access to the color data register.
$0404 - CTW - Color Table Write Register / CTR - Color Table Read Register
Write/Read register.
Access (read/write) to this register causes CTA register to increment.
| Bit(s) | Description |
|---|---|
| 0 - 2 | Blue |
| 3 - 5 | Red |
| 6 - 8 | Green |
| 9 - 15 | (unused) |