CRTC pins RA3, RA4, MA10, MA11 are not connected on CPC.
=== Overscan bits ===
It's possible to use 32KB screen size (used for [[Programming:Overscan|overscan]]) by setting bits 11 and 10 of Register 12 both to 1. Bits MA11 and MA10 of the address generated by the CRTC are not written on the address bus to access video memory; settings both bits to 1 is the only way to cause a carry to bit MA12 when address pass over the end of current video page to change the memory address to the next video page.
.------- REG 12 --------. .------- REG 13 --------.
| | | |
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
.--.--.--.--.--.--.--.--. .--.--.--.--.--.--.--.--.
|X |X | | | | | | | | | | | | | | | |
'--'--'--'--'--'--'--'--' '--'--'--'--'--'--'--'--'
'--.--'--.--'---------------.-----------------'
| | |
| | '------> Offset for setting
| | videoram
| | (1024 positions)
| | Bits 0..9
| |
| '-------------------------> Video Buffer : note (1)
|
'-------------------------------> Video Page : note (2)
note (1) note (2)
.--.--.--------------. .--.--.---------------.
|11|10| Video Buffer | |13|12| Video Page |
|--|--|--------------| |--|--|---------------|
| 0| 0| 16Ko | | 0| 0| 0000 - 3FFF |
|--|--|--------------| |--|--|---------------|
| 0| 1| 16Ko | | 0| 1| 4000 - 7FFF |
|--|--|--------------| |--|--|---------------|
| 1| 0| 16Ko | | 1| 0| 8000 - BFFF |
|--|--|--------------| |--|--|---------------|
| 1| 1| 32Ko | | 1| 1| C000 - FFFF |
'--'--'--------------' '--'--'---------------'
<br>
On CRTC 1, registers 18-30 read as 0, register 31 reads as 0xff.
Details about Reg. 12 and Reg. 13 specifically:
.------- REG 12 --------. .------- REG 13 --------.
| | | |
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
.--.--.--.--.--.--.--.--. .--.--.--.--.--.--.--.--.
|X |X | | | | | | | | | | | | | | | |
'--'--'--'--'--'--'--'--' '--'--'--'--'--'--'--'--'
'--.--'--.--'---------------.-----------------'
| | |
| | '------> Offset for setting
| | videoram
| | (1024 positions)
| | Bits 0..9
| |
| '-------------------------> Video Buffer : note (1)
|
'-------------------------------> Video Page : note (2)
note (1) note (2)
.--.--.--------------. .--.--.---------------.
|11|10| Video Buffer | |13|12| Video Page |
|--|--|--------------| |--|--|---------------|
| 0| 0| 16Ko | | 0| 0| 0000 - 3FFF |
|--|--|--------------| |--|--|---------------|
| 0| 1| 16Ko | | 0| 1| 4000 - 7FFF |
|--|--|--------------| |--|--|---------------|
| 1| 0| 16Ko | | 1| 0| 8000 - BFFF |
|--|--|--------------| |--|--|---------------|
| 1| 1| 32Ko | | 1| 1| C000 - FFFF |
'--'--'--------------' '--'--'---------------'
So, it's possible to use 32KB screen size (used for [[Programming:Overscan|overscan]]) by setting bits 11 and 10 both to 1 (of Register 12). Bits MA11 and MA10 of the address generated by the CRTC are not written on the address bus to access video memory; settings both bits to 1 is the only way to cause a carry to bit MA12 when address pass over the end of current video page to change the memory address to the next video page.
<br>
* On CRTCs 0/1/2, if a Write Only register is read from, "0" is returned. The register accessing scheme on CRTCs 3/4 makes it impossible to happen.
* CRTC types 3 and 4 are identical in every way, except for the unlocking mechanism, split-screen , hardware soft scroll and 8-bit printer port functionalities specific to the ASIC.
* See the document "Extra CPC Plus Hardware Information" for more details.
|}
No matter its type, the CRTC never buffers any of its counters, except for the video pointer MA. A buffer MA' is needed because MA has to be reloaded at each raster line start.
The only value that is saved in a buffer in the CRTC is the video pointer MA because it is reloaded at each raster line start.<br>
R12/R13 is loaded only once per frame, in MA and MA', at the first raster line start of the frame. The == CRTC counter MA is then reloaded with the value of MA' at each raster line start. And at each new character line start, MA' captures the current value of MA.differences ==
The exception is the CRTC 1 for which === MA is reloaded at each raster line start with R12/R13 instead of MA' as long as VCCreload ===0.
On CRTCs 0/3/4, at the first raster line start of the frame, MA and MA' are loaded with R12/R13. Otherwise, MA is loaded with MA'. On CRTC 2, at any raster line start of the frame (including the first one), MA is loaded with MA'. On CRTC 1, at every raster line start of the first character line of the frame (ie. when VCC=0), MA is reloaded with R12/R13 instead of MA'. This discrepancy is a major source of incompatibility if the programmer does not take care of this discrepancy. In demos and games, to make a display be compatible with all CRTCs, program R12/R13 when VCC!=0. This will then take effect at the next frame start.
<br>
== CRTC counter differences =MA buffering === At the end of the display of the last raster line of each character line (ie. when HCC=R1 and VLC=R9), MA' captures the current value of MA. CRTC 2 is the exception, where MA' captures R12/R13 instead of MA at the last line of the frame. <br>
=== VSC (C3h) overflow ===