Changes

== The 6845 Registers CRTC registers ==

== CRTC Differences register differences == In this section I will attempt to identify all the differences between each CRTC.

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.  R12/R13 is loaded only once per frame, in MA and MA', at the first raster line start of the frame. The 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. The exception is the CRTC 1 for which the MA is reloaded at each raster line start with R12/R13 instead of MA' as long as VCC=0. This is a major source of incompatibility if the programmer does not take care of this discrepancy. In demos and games, to make a display compatible with all CRTCs, program R12/R13 when VCC!=0. This will then take effect at the next frame start. <br> == CRTC counter differences == === VSC (C3h) overflow === During a VSYNC on CRTCs 0/3/4, if VSYNC Width (R3h) is changed with a value less than the current VSC, then VSC overflows and will continue to count up to its maximum value (15) before looping back and counting up again until it reaches the new value of R3h. On CRTCs 1/2, the VSYNC width is fixed to 16 characters. It is not possible to modify it. Therefore, VSC cannot be overflowed. <br> === HSC (C3l) overflow === During an HSYNC, if HSYNC Width (R3l) is changed with a value less than the current HSC, then HSC overflows and will continue to count up to its maximum value (15) before looping back and counting up again until it reaches the new value of R3l. The only exception is for CRTC 1 with a value of 0, which cancels the current HSYNC. <br> === VCC (C4) overflow === On all CRTCs, if Vertical Total (R4) is changed with a value less than VCC, then:* if this update was done when VCC < R4, then VCC overflows and will continue to count up to its maximum value (127) before looping back and counting up again until it reaches the new value of R4* if this update was done when VCC = R4, the current character line was already decided to be the last one of the current frame. No update to R4 will make the CRTC change its mind for the current frame The only exception when VCC = R4 is for CRTC 1 with a value of 0, which will cause VCC to overflow. <br> === HCC (C0) overflow === If Horizontal Total (R0) is changed with a value less than the current HCC, then:* on CRTCs 0/1/2, HCC overflows and will count up to its maximum value (255) before looping back and counting up again until it reaches the new value of R0* on CRTCs 3/4, the current line is considered finished and HCC is immediately reset to 0 on the next line <br> === VLC (C9) overflow === If Number of Scan Lines (R9) is changed with a value less than the current VLC, then:* on CRTCs 0/1/2, VLC overflows and will count up to its maximum value (31) before looping back and counting up again until it reaches the new value of R9* on CRTCs 3/4, the current line is considered the last one of this CRTC character and VLC changes to 0 on the next line <br> === VTAC overflow === During vertical adjustment mode, if Vertical Total Adjust (R5) is changed with a value less than the current VTAC, then:* on CRTCs 0/1/2, VTAC overflows and will continue to count up to its maximum value (31) before looping back and counting up again until it reaches the new value of R5* on CRTCs 3/4, the current line is considered the last one of the current frame and vertical adjustment ends <br> === Vertical Adjustment mode === On CRTCs 0/1/2, this mode increments VCC and so VCC goes beyond R4. On CRTCs 3/4, this mode does not increment VCC and so VCC stays equal to R4.