Article 53192 of comp.sys.cbm:
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From: csbruce@ccnga.uwaterloo.ca (Craig Bruce)
Subject: Looking for hard-to-find knowledge on the VDC
Sender: news@undergrad.math.uwaterloo.ca (news spool owner)
Message-ID: <DqLGBK.nwH@undergrad.math.uwaterloo.ca>
Date: Sun, 28 Apr 1996 22:40:32 GMT
Nntp-Posting-Host: ccnga.uwaterloo.ca
Organization: University of Waterloo, Canada (eh!)

Yo!

>I am looking for a way to get a "double-height" interlaced screen on the 
>128 (with 64K VDC RAM). I have seen a program (in Compute's Gazette, I 
>believe) that enables 80x50 text mode, but I don't know exactly how. How 
>could someone do this, and could the same technique be used in bitmap 
>mode?

The ACE operating system supports the 80x50 (up to 80x51, actually) mode.
The VDC memory is used as follows:

$0000 = screen matrix
$1000 = color matrix
$2000 = uppercase/graphics character set
$3000 = lowercase character set

You have to change the color address from its default $0800.  Here is the
ACE code that goes into 80x.A interlaced mode:

vdcRegSaves  .buf 5
vdcRegSave5  .buf 1
vdcRegSave7  .buf 1

vdcRegSaveIndex .byte 0,4,8,9,6,5,7

vdcVerticalCrossover = *  ;( .A=rows ) : .A=vdcReg5, .Y=vdcReg7
   cmp #51
   bcc +
   lda #51
+  pha
   ldy #6
-  lda vdcRegSaveIndex,y
   tax
   lda vdcRegFiftyRows,y
   jsr vdcWrite
   dey
   bpl -
   pla
   ldx #6
   jsr vdcWrite
   tay
   clc
   adc #1
   lsr
   and #%11111110
   clc
   adc #27
   cpy #50
   bcc +
   lda #53
+  ldx #7
   jsr vdcWrite
   rts
   vdcRegFiftyRows .byte $80,$38,$ff,$e8,51,$06,$35

The following ML code is extracted from my (standalone) VBM image viewer
program for the C128.  It will set up the screen for 640x491 interlaced
monochrome bitmapped graphics.

.$1329  [a0 00   ]  ldy #$00
.$132b  [be 4a 13]  ldx $134a,y
.$132e  [b9 5e 13]  lda $135e,y
.$1331  [e0 19   ]  cpx #$19
.$1333  [d0 08   ]  bne $133d
.$1335  [20 1b 13]  jsr $131b     ;read VDC register #.X into .A
.$1338  [29 0f   ]  and #$0f
.$133a  [19 5e 13]  ora $135e,y
.$133d  [20 0f 13]  jsr $130f     ;write value .A into VDC register #.X
.$1340  [c8      ]  iny
.$1341  [c0 14   ]  cpy #$14
.$1343  [90 e6   ]  bcc $132b

$134a: 00 01 02 04 05 06 07 08 09 18 19 1b 1c 14 15 0c 0d 1a 0c 0d

$135e: 7e 50 66 4c 06 4c 47 03 06 00 80 00 10 a6 e0 00 e0 00 00 48


>Also, I read somewhere that 8x1 (that's right, 1) color cell mode 
>_should_ be possible. It didn't say whether it's ever been done, or how 
>to do it. Anyone know?

Yes, the bitmap would take 80x200 bytes = 16000 bytes.  The color map would
take another 16000 bytes, for a total of 32000 bytes.

In fact, if there were 4x1 display cells, then it would all fit in 64000
bytes.

I've tried to get 8x1 color cells before, but the hardware freaks out if
I try anything beneath 8x2.  I'd also like to know how to get characters
that are fewer than eight pixels wide, say 4x8 characters, without using
a bitmap.

Keep on Hackin'!

-Craig Bruce
csbruce@ccnga.uwaterloo.ca
"I just got paid and I want to get ..." - Bud, Married... With Children