x86 programming for Ms Dos Computers in CGA , EGA and VGA

The IBM 8086 took the world by storm!

Although inferior to the 68000 - the x86 soon took over, and is the basis for all the computers we have today...

Starting with the 8086, and soon moving to the 286, 386 and so on... each iteration has added more commands and power, and along the way the PC has gained functionality...

In these tutorials, we'll take a look at the early basic machines, and learn how we can use Assembly to write games that can be used in MSDos via DosBox!

We'll cover 3 graphics modes...

CGA	320x200 4 color (Cyan,Magenta,White or Green,Red,Yellow)
EGA	320x200 16 color fixed palette
VGA	320x200 256 color palette

If you want to learn 8086 get the Cheatsheet! it has all the 8086 commands, It will help you get started with ASM programming, and let you quickly look up commands when you get confused!

Useful Documents

MASM61PROGUIDE - Microsoft Assembler guide
MASM - Microsoft's Dos based assembler
80x86 IBM PC and Compatible Computers - Programming guide
8086/186 - Intel CPU manual

CGA color Palette

	Palette 0 Dark	Palette 0 Bright	Palette 1 Dark	Palette 1 Bright
0
1
2
3

EGA color Palette

The EGA system uses 16 colors - it's 320x200 can only use a palette of 16 colors (the same colors!)

Each color has a logical number from 0-16, and a Hardware number (used for palette definitions) from 0-63

0 - 0	1 - 1	2 - 2	3 - 3	4 - 4	5 - 5	6 - 20	7 - 7
8 - 56	9 - 57	10 - 58	11 - 59	12 - 60	13 - 61	14 - 62	15 - 63

Screen Layouts

CGA is a 2bpp screen mode, it's memory is at B800:0000h

EGA use 4 bitplanes, it's address is A000:0000... to change the bitplanes we need to OUT to 03C4h

VGA is a 8bpp screen mode, it's memory is at A000:0000h

Bits

Screen Mode

Bits per pixel

Pixels per byte

Address

Plane Mask

CGA - 4

2 bpp

B800:0000h

EGA - 13

4 bitplanes

A000:0000h

out 03c4h,0102h

p0-b7

p0-b6

p0-b5

p0-b4

p0-b3

p0-b2

p0-b1

p0-b0

out 03c4h,0202h

p1-b7

p1-b6

p1-b5

p1-b4

p1-b3

p1-b2

p1-b1

p1-b0

out 03c4h,0302h

p2-b7

p2-b6

p2-b5

p2-b4

p2-b3

p2-b2

p2-b1

p2-b0

out 03c4h,0402h

p3-b7

p3-b6

p3-b5

p3-b4

p3-b3

p3-b2

p3-b1

p3-b0

out 03c4h,0F02h

All-b7

All-b6

All-b5

All-b4

All-b3

All-b2

All-b1

All-b0

VGA

8bpp

A000:0000h

Beeper Sound Ports

Port	Modes	Purpose	Bits	Notes
0040	RW	PIT counter 0, counter divisor (XT, AT, PS/2)	CCCCCCCC	Send L/H Pair
0041	RW	PIT counter 1, RAM refresh counter (XT, AT)	CCCCCCCC	Send L/H Pair
0042	RW	PIT counter 2, cassette & speaker (XT, AT, PS/2)	CCCCCCCC	Send L/H Pair
0043	RW	PIT mode port, control word register for counters 0-2	CCAAMMMS	C=Counter select (0-2), A=counter Access, M=counter Mode (0-5), S=counter Style (0=16 bit 1=BCD)
0061	W	PPI Programmable Peripheral Interface 8255 (XT only)	----PPST	P= parity checks S=Speaker enable T=speaker Timer enable
0061	R	KB controller port B control register (ISA, EISA)	EETDPPST	E=errors T=Timer D=Detect P= parity checks S=Speaker enable T=speaker Timer enable

Adlib OPL2 Registers

The ADLIB sound card usesd OPL2, which is also supported by the full SoundBlaster range, it uses a range of registers to make its sounds, each sound channel is formed by a combination of two Operators

NOTE: OPL3 doubled the number of registers, with an 'Advanced' set... for simplicity (and my sanity) we'll just be covering the basic OPL2 set, which are supported by OPL3 as well!

There are a total of up to 9 sound channels... each sound is the combination of two "OP signals"... we should set both to get a sound from a channel! How the OPs are combined is defined by bit 0 of registers C0h-C8h... see the pdf documents for more info.

Channel Signal	1		2		3		4		5		6		7 (Ryt)		8 (Ryt)		9 (Ryt)
OP1 Slot 1 Signal	1		2		3		7		8		9		13		14		15
OP2 Slot 2 Signal		4		5		6		10		11		12		16		17		18
Register settings for slot	20		21		22		28		29		2A		30		31		32
		23		24		25		2B		2C		2D		33		34		35
	40		41		42		48		49		4A		50		51		52
		43		44		45		4B		4C		4D		53		54		55
	60		61		62		68		69		6A		70		71		72
		63		64		65		6B		6C		6D		73		74		75
	80		81		82		88		89		8A		90		91		92
		83		84		85		8B		8C		8D		93		94		95
	E0		E1		E2		E8		E9		EA		F0		F1		F2
		E3		E4		E5		EB		EC		ED		F3		F4		F5
Register settings for the channel	A0		A1		A2		A3		A4		A5		A6		A7		A8
	B0		B1		B2		B3		B4		B5		B6		B7		B8
	C0		C1		C2		C3		C4		C5		C6		C7		C8

Channels 7,8,9 can be toggled as Rhythm effects by setting bit 5 of 0BDh to 1

In this mode bits 0-4 of 0BDh will 'fire' the effects... each effect uses some of the signal slots, the registers for this slot will need to be set up as usual

BDh bits %DDRBSTCH

R=Rhythm enabled (channel 7-9 no longer normal FM sound)

Bit / Rhythm sound	OP / Signal Slots used
B=Bass	13 & 16
S=Snare	17
T=Tom	15
C=Cymbal	18
H=Hihat	14

Sound over time

The OPs define how the sound level changes over time... K-On and K-Off mimic the way piano keys work.. when the key is struck the sound will start (Attack), and fade slowly (Decay) to a constant tone (Sustain), when the key is lifted, it will fade quickly (Release)

Adlib OPL2 Registers and bits

Register	Details	Bits	Details
01h	Test	--WDDDDD	W=Wave select Enable (opl2) / D=Test Data
02h	Timer 1 Setting 80-20.4us	TTTTTTTT	T=Timer
03h	Timer 2 Setting 320-82 us	TTTTTTTT	T=Timer
04h	Timer 1/2 control	RMM---SS	R=Reset M=Mask S=?
08h	Speech Synth / Keyboard Split NoteSel	CS------	C=CSM Speech synth mode / S=note Select
20h - 35h	Multi / Key Scale Rate / EG-Type Tone / Vibrato / AM modulation	AVEKMMMM	A=AM V=VIB E=EG-Typ K=KSR M=Multiple
40h - 55h	Total Level / Key Scale Level	KKTTTTTT	K=KeyScaleLevel T=Total Level (0=loud)
60h - 75h	Decay Rate / Attack Rate	AAAADDDD	A=Attack (0=slow) D=Decay (0=slow)
88h - 95h	Release Rate / Sustain Level	SSSSRRRR	S=Sustain (0=loud) R=Release (0=slow)
A0h - A8h	F number	FFFFFFFF	F=Fnumber L
B0h - B8h	Block / K-ON	--KBBBFF	F=Fnumber H B=Block K=K-on
BDh	Rhythm mode (Chn 7-9) / Vibrato Depth / AM Depth	DDRBSTCH	D=Depth (AM/VIB) R=Rhythm B=Bass(13,16) S=Snare(17) T=Tom(15) C=Cymbal(18) H=Hihat(14)
C0h - C8h	FeedBack factor / C=Connection sine/fm	----FFFC	F=Feedback C=Connection (Op combination mode)
E0h - F5h	Wave Select	------WW	WW=Wave Select

(Address port Read)	Status Reg	IFF-----	I=IRQ F=Flag

Useful ADLIB docs:

yamaha_ymf262 - OPL3 Manual (Adlib Gold / SB16)
YM3812 - OPL2 Manual (adlib)
ym3625 - OPL(1) manual
Soundblaster - Soundblaster programming guide
Adlib Programming - Adlib programming guide