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

Amstrad PC1512 IBM Compatible

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 7 6 5 4 3 2 1 0
CGA - 4 2 bpp 4 B800:0000h
b1 b0 b1 b0 b1 b0 b1 b0
EGA - 13 4 bitplanes 8 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 1 A000:0000h
b7 b6 b5 b4 b3 b2 b1 b0

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


 

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your local Amazon website!
Click here for more info!