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Filename Latest commit message Latest commit date
Jeff Parsons badaaa307e Fix pushing/popping at the limits of the stack segment
2015-06-02 15:39:13 -07:00
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README.md Fix pushing/popping at the limits of the stack segment 2015-06-02 15:39:13 -07:00

README.md

Debugging the IBM VGA ROM

The IBM VGA ("Video Graphics Array") standard was introduced as part of the IBM PS/2 line of computers; it was not a feature you could purchase or install in older PC, XT or AT-compatible machines. In fact, full VGA support was not even available in all PS/2 models. Of the first four PS/2 models -- the 8086-based Model 30, the 80286-based Model 50 and Model 60, and the 80386-based Model 80 -- VGA support was available only in the three higher-end models. The Model 30 came with MCGA ("Multicolor Graphics Array") video hardware that supported a subset of VGA modes (eg, 640x480 2-color and 320x200 256-color graphics).

It wasn't until October 1987 that IBM finally introduced an 8-bit ISA card that brought VGA capability to older PCs. The card was called the IBM PS/2 Display Adapter. However, I think the name is a bit confusing, since the card could only be used in PC, XT, and AT-compatible systems. I'll refer to it here simply as the IBM VGA.

The VGA ROM used here is assumed to have come from an original IBM VGA. It's unknown if IBM ever made any revisions to the VGA ROM. With the introduction of the PS/2 family and the VGA, IBM decided to no longer publish the source code for its ROMs, so I've created some assemblable source code from the IBM VGA ROM here.

I've finally started debugging a machine configuration that uses the IBM VGA ROM. Since the VGA and the 80386 are contemporaries, I'm using an 80386 machine configuration. However, I don't expect the IBM VGA ROM to require any 80386 support or PS/2-specific features.

The first problem I ran into was here:

;
;   Initialize the ROM BIOS Video Mode Options byte @40:0087 (default to color and 256Kb of RAM)
;
mov	byte [0x487],0x60	; 0000008D
;
;   The x100 subroutine alternately enables port 0x3B? and 0x3D? decoding, verifying that there is
;   no response on opposing ports 0x3D? and 0x3B?, respectively; otherwise, it assumes that another
;   video card must exist and attempts to select co-existing settings for the VGA.  For example, if
;   there is an unexpected response on the color ports, the VGA ROM will default to mono operation.
;
call	x100			; 00000092

The Video component installs I/O port handlers for all possible I/O ranges; when a range isn't being used, the associated I/O operations are redirected to a dummy Card, so that the active Card isn't affected. Here, however, that was insufficient. If the VGA is the only installed video card, the VGA ROM expects NO RESPONSE on inactive CRTC ports. So I've changed the CRTC I/O handlers to check the Card's fActive flag. This seems like a safe and logical change, but I still have to check for backward-compatibility issues with older ROMs.

The VGA ROM programs the card for the first time here:

EAX=00000000 EBX=00001642 ECX=00000004 EDX=00004AE8 
ESP=000000EA EBP=FFFF00F0 ESI=00006000 EDI=000003D4 
SS=0030 DS=0000 ES=C000 FS=0000 GS=0304 PS=00000246 V0 D0 I1 T0 S0 Z1 A0 P1 C0 
C000:00CB E85910          CALL     1127                 ;cycles=5
videoVGA.outPort(0x03C4,SEQ.INDX,0x00) @C000:112F
videoVGA.outPort(0x03C5,SEQ.RESET,0x01) @C000:112F
videoVGA.outPort(0x03C4,SEQ.INDX,0x01) @C000:1204
videoVGA.outPort(0x03C5,SEQ.CLK,0x29) @C000:1204
videoVGA.outPort(0x03C4,SEQ.INDX,0x02) @C000:1204
videoVGA.outPort(0x03C5,SEQ.MAPMASK,0x0F) @C000:1204
videoVGA.outPort(0x03C4,SEQ.INDX,0x03) @C000:1204
videoVGA.outPort(0x03C5,SEQ.CHARMAP,0x00) @C000:1204
videoVGA.outPort(0x03C4,SEQ.INDX,0x04) @C000:1204
videoVGA.outPort(0x03C5,SEQ.MODE,0x06) @C000:1204
videoVGA.outPort(0x03C2,MISC,0x63) @C000:1143
videoVGA.outPort(0x03C4,SEQ.INDX,0x00) @C000:1149
videoVGA.outPort(0x03C5,SEQ.RESET,0x03) @C000:1149
videoVGA.outPort(0x03D4,CRTC.INDX,0x11) @C000:1151
videoVGA.outPort(0x03D5,CRTC.VERT_RETRACE_END,0x00) @C000:1151
videoVGA.outPort(0x03D4,CRTC.INDX,0x00) @C000:1204
videoVGA.outPort(0x03D5,CRTC.HORZ_TOTAL,0x5F) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x01) @C000:1204
videoVGA.outPort(0x03D5,CRTC.HORZ_DISP_END,0x4F) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x02) @C000:1204
videoVGA.outPort(0x03D5,CRTC.HORZ_BLANK_START,0x50) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x03) @C000:1204
videoVGA.outPort(0x03D5,CRTC.HORZ_BLANK_END,0x82) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x04) @C000:1204
videoVGA.outPort(0x03D5,CRTC.HORZ_RETRACE_START,0x55) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x05) @C000:1204
videoVGA.outPort(0x03D5,CRTC.HORZ_RETRACE_END,0x81) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x06) @C000:1204
videoVGA.outPort(0x03D5,CRTC.VERT_TOTAL,0xBF) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x07) @C000:1204
videoVGA.outPort(0x03D5,CRTC.CRTC_OVERFLOW,0x1F) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x08) @C000:1204
videoVGA.outPort(0x03D5,CRTC.PRESET_ROW_SCAN,0x00) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x09) @C000:1204
videoVGA.outPort(0x03D5,CRTC.MAX_SCAN_LINE,0x40) @C000:1204
removeCursor(): removed from 0,0
videoVGA.outPort(0x03D4,CRTC.INDX,0x0A) @C000:1204
videoVGA.outPort(0x03D5,CRTC.CURSOR_START,0x00) @C000:1204
checkCursor(): cursor moved from -1 to 0
videoVGA.outPort(0x03D4,CRTC.INDX,0x0B) @C000:1204
videoVGA.outPort(0x03D5,CRTC.CURSOR_END,0x00) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x0C) @C000:1204
videoVGA.outPort(0x03D5,CRTC.START_ADDR_HI,0x00) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x0D) @C000:1204
videoVGA.outPort(0x03D5,CRTC.START_ADDR_LO,0x00) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x0E) @C000:1204
videoVGA.outPort(0x03D5,CRTC.CURSOR_ADDR_HI,0x00) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x0F) @C000:1204
videoVGA.outPort(0x03D5,CRTC.CURSOR_ADDR_LO,0x00) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x10) @C000:1204
videoVGA.outPort(0x03D5,CRTC.VERT_RETRACE_START,0x9C) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x11) @C000:1204
videoVGA.outPort(0x03D5,CRTC.VERT_RETRACE_END,0x8E) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x12) @C000:1204
videoVGA.outPort(0x03D5,CRTC.VERT_DISP_END,0x8F) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x13) @C000:1204
videoVGA.outPort(0x03D5,CRTC.OFFSET,0x28) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x14) @C000:1204
videoVGA.outPort(0x03D5,CRTC.UNDERLINE,0x1F) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x15) @C000:1204
videoVGA.outPort(0x03D5,CRTC.VERT_BLANK_START,0x96) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x16) @C000:1204
videoVGA.outPort(0x03D5,CRTC.VERT_BLANK_END,0xB9) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x17) @C000:1204
videoVGA.outPort(0x03D5,CRTC.MODE_CTRL,0xE3) @C000:1204
videoVGA.outPort(0x03D4,CRTC.INDX,0x18) @C000:1204
videoVGA.outPort(0x03D5,CRTC.LINE_COMPARE,0xFF) @C000:1204
videoVGA.inPort(0x03DA,STATUS1): 0x00 @C000:1167
videoVGA.outPort(0x03C0,ATC.INDX,0x10) @C000:116C
videoVGA.outPort(0x03C0,ATC.MODE,0x01) @C000:1171
videoVGA.outPort(0x03C0,ATC.INDX,0x12) @C000:1174
videoVGA.outPort(0x03C0,ATC.PLANES,0x0F) @C000:1179
videoVGA.outPort(0x03C0,ATC.INDX,0x13) @C000:117C
videoVGA.outPort(0x03C0,ATC.HORZPAN,0x00) @C000:1181
videoVGA.outPort(0x03CE,GRC.INDX,0x00) @C000:1204
videoVGA.outPort(0x03CF,GRC.SRESET,0x00) @C000:1204
videoVGA.outPort(0x03CE,GRC.INDX,0x01) @C000:1204
videoVGA.outPort(0x03CF,GRC.ESRESET,0x00) @C000:1204
videoVGA.outPort(0x03CE,GRC.INDX,0x02) @C000:1204
videoVGA.outPort(0x03CF,GRC.COLRCMP,0x0F) @C000:1204
videoVGA.outPort(0x03CE,GRC.INDX,0x03) @C000:1204
videoVGA.outPort(0x03CF,GRC.DATAROT,0x00) @C000:1204
videoVGA.outPort(0x03CE,GRC.INDX,0x04) @C000:1204
videoVGA.outPort(0x03CF,GRC.READMAP,0x00) @C000:1204
videoVGA.outPort(0x03CE,GRC.INDX,0x05) @C000:1204
videoVGA.outPort(0x03CF,GRC.MODE,0x08) @C000:1204
setAccess(0x0210)
videoVGA.outPort(0x03CE,GRC.INDX,0x06) @C000:1204
videoVGA.outPort(0x03CF,GRC.MISC,0x05) @C000:1204
setMode(0x0010)
setMode(16): removing 0x00008000 bytes from 0x000B8000
setMode(16): adding 0x00010000 bytes to 0x000A0000
videoVGA.outPort(0x03CE,GRC.INDX,0x07) @C000:1204
videoVGA.outPort(0x03CF,GRC.COLRDC,0x0F) @C000:1204
videoVGA.outPort(0x03CE,GRC.INDX,0x08) @C000:1204
videoVGA.outPort(0x03CF,GRC.BITMASK,0xFF) @C000:1204

EAX=0000FF09 EBX=00001642 ECX=00000000 EDX=000003DA 
ESP=000000EA EBP=FFFF00F0 ESI=00006000 EDI=000003D4 
SS=0030 DS=0000 ES=C000 FS=0000 GS=0304 PS=00000246 V0 D0 I1 T0 S0 Z1 A0 P1 C0 
C000:00CE 32C0            XOR      AL,AL                ;cycles=1031

Now that the card was programmed for the first time, I used the d video Debugger command to dump the video card's state:

BIOSMODE: 0x10
CRTC[0x00]: HORZ_TOTAL         0x5F
CRTC[0x01]: HORZ_DISP_END      0x4F
CRTC[0x02]: HORZ_BLANK_START   0x50
CRTC[0x03]: HORZ_BLANK_END     0x82
CRTC[0x04]: HORZ_RETRACE_START 0x55
CRTC[0x05]: HORZ_RETRACE_END   0x81
CRTC[0x06]: VERT_TOTAL         0xBF
CRTC[0x07]: CRTC_OVERFLOW      0x1F
CRTC[0x08]: PRESET_ROW_SCAN    0x00
CRTC[0x09]: MAX_SCAN_LINE      0x40
CRTC[0x0A]: CURSOR_START       0x00
CRTC[0x0B]: CURSOR_END         0x00
CRTC[0x0C]: START_ADDR_HI      0x00
CRTC[0x0D]: START_ADDR_LO      0x00
CRTC[0x0E]: CURSOR_ADDR_HI     0x00
CRTC[0x0F]: CURSOR_ADDR_LO     0x00
CRTC[0x10]: VERT_RETRACE_START 0x9C
CRTC[0x11]: VERT_RETRACE_END   0x8E
CRTC[0x12]: VERT_DISP_END      0x8F
CRTC[0x13]: OFFSET             0x28
CRTC[0x14]: UNDERLINE          0x1F
CRTC[0x15]: VERT_BLANK_START   0x96
CRTC[0x16]: VERT_BLANK_END     0xB9
CRTC[0x17]: MODE_CTRL          0xE3
CRTC[0x18]: LINE_COMPARE       0xFF*
   STATUS1: 0x00
   ATCDATA: false
 ATC[0x00]: PAL00   0x??
 ATC[0x01]: PAL01   0x??
 ATC[0x02]: PAL02   0x??
 ATC[0x03]: PAL03   0x??
 ATC[0x04]: PAL04   0x??
 ATC[0x05]: PAL05   0x??
 ATC[0x06]: PAL06   0x??
 ATC[0x07]: PAL07   0x??
 ATC[0x08]: PAL08   0x??
 ATC[0x09]: PAL09   0x??
 ATC[0x0A]: PAL0A   0x??
 ATC[0x0B]: PAL0B   0x??
 ATC[0x0C]: PAL0C   0x??
 ATC[0x0D]: PAL0D   0x??
 ATC[0x0E]: PAL0E   0x??
 ATC[0x0F]: PAL0F   0x??
 ATC[0x10]: MODE    0x01
 ATC[0x11]: OVRSCAN 0x??
 ATC[0x12]: PLANES  0x0F
 ATC[0x13]: HORZPAN 0x00*
 GRC[0x00]: SRESET  0x00
 GRC[0x01]: ESRESET 0x00
 GRC[0x02]: COLRCMP 0x0F
 GRC[0x03]: DATAROT 0x00
 GRC[0x04]: READMAP 0x00
 GRC[0x05]: MODE    0x08
 GRC[0x06]: MISC    0x05
 GRC[0x07]: COLRDC  0x0F
 GRC[0x08]: BITMASK 0xFF*
 SEQ[0x00]: RESET   0x03*
 SEQ[0x01]: CLK     0x29
 SEQ[0x02]: MAPMASK 0x0F
 SEQ[0x03]: CHARMAP 0x00
 SEQ[0x04]: MODE    0x06
	  FEAT: 0x00
	  MISC: 0x63
   STATUS0: 0x00
   LATCHES: 0x00
	ACCESS: 0x0210
Use 'dump video buffer' to dump video memory

If you've noticed that none of the ATC palette registers were programmed, well, that's apparently by design.

@jeffpar
June 1, 2015