VCC only displays 199 rows on a 160x200x16 color graphics screen

[CoCoPaul70]

VCC 2.1.9.2 seems to have a bug where it only displays 199 rows on a graphics screen that has been set to 160x200x16 colors. All 200 rows shows properly on real hardware, XRoar and Mame.

Here is the assembly code I used to set the graphics screen:

			lda		#%01100000              	; set Initialization register: MMU enabled; GIME IRQ enabled
			sta		$ff90
			
			ldd		#$80b6                 	 	; set Video Mode and Video Resolution registers $ff98 and $ff99
			std		$ff98                   	        ; $ff98 set to %10000000: graphics mode enabled, one lines per row
										; $ff99 set to %10110110: 200 scan lines, 80 bytes per row (160 horiz rez), 16 colors

[ejaquay]

I found this in Lomont:

Due to a design error in the GIME, the "200-line" mode only displays 199 lines of active video on the screen. If you do the BASIC pokes for 25 lines on the WIDTH 40 and WIDTH 80 screens, you will see the blinking underscore cursor disappear at the bottom line. If the graphic screens are poked for 200 lines, the bottom-most line will be 198, not 199. Try it and see. (Rodney V Hamilton)

This means this bug is in real hardware and VCC is simply emulating it. Mame and Xroar seem to be emulating a GIME-X which perhaps fixed the original GIME bug?

[ejaquay]

Changed this to a feature request.

[CoCoPaul70]

Ah ha! That makes sense, and now that you point out the GIME bug, bells are ringing that I had once known that (but had forgotten in my old age). Indeed, my real hardware CoCo 3 has the GIME-X installed in it. Thank you for checking into this!