[MSX] CMP

[Gerald Stap]

Hi,

Does anyone have any information regarding the encoding and decoding of 
screen 5 pictures in CMP format? CMP is the format that DD-GRAPH/AGE 
uses for compressed screen 5 pictures and is seems to be very efficient. 
But I've never seen any document or article about how CMP works.

I've got this snippet of code. Don't know where it's from, who wrote it, 
or if it actually works. (It's been on my harddrive since 1994 if my 
clock chip was set correctly). I'm more interested in the theory behind 
CMP so I can write my own encoder/decoder.

Note: Comments are a mix of dutch/english
=============
;  UNCRUNCH ROUTINE   T&E SOFT
;
;   IN: HL= RAMADRES CRUNCH DATA
;       DE= COORDINATEN IN VRAM
;       A = VRAM PAGE (0,1,2,3)
;
CMPER:: JP    USTART
;
UVARIA: DB    0               ;&H4EC0          ;&HE000          ; 1 BYTE
UTABEL: DB    0,0,0,0,0,0,0,0 ;&H4EC1          ;&HE001          ; 8 BYTES
UTABE2: DW    0,0,0,0,0,0,0,0,0,0 ;&H4EC9          ;&HE009          ; 
MAX. 256 B
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
       DW    0,0,0,0,0,0,0,0,0,0
;
USKIP   EQU   0FEH
UVDP    EQU   98H
;
UROUT1: XOR   A
       SLA   B
       JR    NC,X1
       LD    A,(HL)
       INC   HL
X1:     DEC   C
       RET   NZ
       PUSH  AF
       LD    A,(DE)
       LD    B,A
       LD    A,E
       CP    UTABE2 MOD 256
       JR    NZ,X2
       DEFB  USKIP           ; PUSH AF won't be executed now !
ROUTIN: PUSH  AF
       LD    C,(HL)
       INC   HL
       LD    DE,UTABEL
       LD    B,8
X4:     XOR   A
       SLA   C
       JR    NC,X3
       LD    A,(HL)
       INC   HL
X3:     LD    (DE),A
       INC   DE
       DJNZ  X4
       LD    DE,UTABEL
       LD    A,(DE)
       LD    B,A
X2:     INC   DE
       LD    C,8
       POP   AF
       RET

USTART: LD    B,A
       SRL   A
       LD    (UVARIA),A
       LD    A,D
       LD    D,E
       LD    E,A
       XOR   A
       SRL   D
       RRA
       ADD   A,E
       LD    E,A
       JR    NC,X5
       INC   D
X5:     BIT   0,B
       JR    Z,X6
       SET   7,D
X6:     PUSH  DE
       LD    B,(HL)
       INC   HL
       LD    C,(HL)
       INC   HL
       LD    A,(HL)
       INC   HL
       EX    AF,AF
       PUSH  BC
       CALL  ROUTIN
       EXX
       POP   BC
       POP   HL
       DEFB  USKIP           ; EX AF,AF won't be executed now
X8:     EX    AF,AF
       PUSH  BC
       LD    A,(UVARIA)
       CALL  USETWR
       LD    DE,UTABE2
X7:     EXX
       CALL  UROUT1
       EXX
       LD    (DE),A
       INC   DE
       OUT   (C),A
       DJNZ  X7
       EI
       LD    C,128
       ADD   HL,BC
       POP   BC
       DEC   C
       RET   Z
       EX    AF,AF
       DEC   A
       JP    NZ,X8
X9:     PUSH  BC
       LD    A,(UVARIA)
       CALL  USETWR
       LD    DE,UTABE2
       EX    DE,HL
X10:    EXX
       CALL  UROUT1
       EXX
       XOR   (HL)
       LD    (HL),A
       INC   HL
       OUT   (C),A
       DJNZ  X10
       EI
       EX    DE,HL
       LD    C,128
       ADD   HL,BC
       POP   BC
       DEC   C
       JP    NZ,X9
       LD    B,E
       LD    B,C
       LD    D,E
       RET
;
USETWR: DI
       PUSH  BC
       PUSH  HL
       LD    C,H
       RL    C
       RLA
       RL    C
       RLA
       OUT   (UVDP+1),A
       LD    A,8EH
       OUT   (UVDP+1),A
       LD    C,UVDP+1
       OUT   (C),L
       RES   7,H
       SET   6,H
       OUT   (C),H
       POP   HL
       POP   BC
       LD    C,UVDP
       RET
EINDE:  NOP
;       EINDE UNCRUNCH ROUTINE
       END
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