Hello,

I have a situation with graphics on the system with integrated board that supports Plantronics modes. I'm unable to engage it from my own code, yet I have working examples of other software together with the source. I'm missing something, and I don't know why.

Using latest FreeGEM and sdpla.cga driver, which is available in source format as pla_drv.a86, I can see following is performed;

1  	mov	ah,0
2  	mov	al,5			;hi-res graphics
3  	int	screen
4.
5.
6.
7	mov	dx, 03DDh
8	mov	al, 20h			; 640x200 4-colour mode
9	out	dx, al
10	mov	ax, 1			; Switch to palette 1
11	call	setpal
12.
13.
14.
15setpal:
16.
17.
18.
19	mov	bl, al
20	mov	bh, 1
21	mov	ah, 0Bh
22	int	screen

I've also contacted the author of the driver, 'hi-res graphics' comment was leftover from template code, and there is nothing special about this initialization sequence, it's per Plantronics documentation.

So I tried to replicate it in C :

1union REGS reg;
2reg.h.ah = 0;
3reg.h.al = 5;
4int86(0x10, &reg, &reg);
5
6outp(0x03DD, 0x20);
7
8reg.h.ah =  0x0B;
9reg.h.bh = 1;
10reg.h.bl = 0;
11int86(0x10, &reg, &reg);

The following code puts video at 640x200 but BIOS still outputs 'fat letters' which is correct because it's in CGA mode 5...meaning the Plantronics register worked.

However if I try to write pixels to two planes like this

1unsigned char far *plane1 = (unsigned char far *)MK_FP(0xB800,0x0000);
2unsigned char far *plane2 = (unsigned char far *)MK_FP(0xBC00,0x0000);
3
4unsigned char p1h = 0x55;
5unsigned char p2h = 0x33;
6
7for(i = 0; i < 80; i++) { 
8    memcpy(plane1 + i, &p1h, 1);
9    memcpy(plane2 + i, &p2h, 1);
10}

There is no effect in second memcpy. Expected is one row filled with repeated colour combinations. I get on-off pattern, which is standard CGA 640x200 behaviour.

If I adapt this code to work at 320x200x16 I get same thing. Writing repeatedly FF AA 55 00 and E4 E4 E4 E4 to banks row 0 should give me a repeated 16 colour gradient. I get 4 colours there.

I'm attaching the source of the GEM driver that works correctly. I'm left without ideas and looking for tips.