You remember MMPF #5 ? It included a big article about the special instructions offered by the Z80. But the part about the codes beginning with #ED must be updated 'cause l've found some errors in my codes-table. So is there the new codes-table:
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 4 | IN B,
(C) | OUT
(C),B | SBC
(ADR),BC | LD
HL,BC | NEG | RETN | IM 0 | LD I,A |
| 5 | IN D,
(C) | OUT
(C),D | SBC
HL,DE | LD
(ADR),DE | *NEG | *RETN | IM 1 | LD A,I |
| 6 | IN H,
(C) | OUT
(C),H | SBC
HL,HL | LD
(ADR),HL | *NEG | *RETN | *RESET | RRD |
| 7 | *IN F,
(C) | *NOP | SBC
HL,SP | LD
(ADR),SP* | NEG | *RETN | *NOP | *NOP |
| 8 | 9 | A | B | C | D | E | F |
| 4 | IN C,
(C) | OUT
(C),C | ADC
HL,BC | LD
BC,(ADR) | *NEG | RETI | *NOP ? | LD R,A |
| 5 | IN E,
(C) | OUT
(C),E | ADC
HL,DE | LD
DE,(ADR) | *NEG | *RETI | IM 2 | LD A,R |
| 6 | IN L,
(C) | OUT
(C),L | ADC
HL,HL | LD
HL,(ADR) | *NEG | *RETI | *RESET | RLD |
| 7 | IN A,
(C) | OUT
(C),A | ADC
HL,SP | LD
SP,(ADR) | *NEG | *RETI | *RESET | *NOP |
The main modification concerns the IO instructions: you can see that there is an instruction named IN F,(C) !! As you can guess, its function is to import a value from the IO port adressed by BC and put it into the Flags register. So you can test some bits without using BIT or AND if this bit is at the position of one of the four testable flags (S,Z,V,C). For example, to wait for VSync, you can make: LD B,#F5
VBL IN F,(C)
JR NC,VBLBut, you will tell me, what use can be those special codes, except of these garbage-tricks ? Well, it will help you if you wanna build your own protections or... crack (OH !!!) other ones. A typical example is the loader of Super Cauldron (Elmsoft's new game). It begins with:
#2FD1 LD HL,#3000
#2FD4 LD BC,#500
#2FD7 LD IX,#1234
#2FDB DI
#2FDC XOR A
#2FDD LD R,A
#2FDF LOOP *INC IXL
#2FE1 LD A,R
#2FE3 *XOR IXH
#2FE5 XOR (HL)
#2FE6 *DEC IXH
#2FE8 *NEG
#2FEA DEC BC
#2FEB *NEG
#2FED *XOR IXL
#2FEF XOR C
#2FF0 *INC IYL
#2FF2 LD (HL),A
#2FF3 *DEC IYH
#2FF5 INC HL
#2FF6 *INC IYL
#2FF8 LD A,B
#2FF9 *DEC IYH
#2FFB OR C
#2FFC *LD A,IYL
#2FFE JR NZ,LOOP
#3000 ...If you don't know about the special codes, you can't decode the loader. The technique to decode it is to simulate the variations of the R register with another register (here E is used). In this case you can forget the two NEGs (who act in opposition of each other) and the IY register manipulations 'cause IY has no function in this decoding (except to increase R of 2). The solution to this decoding problem is then:
ORG #A000 ; DEC BC
LD HL,#3000 ; DEFB #DD
LD BC,#500 ; XOR L
LD IX,#1234 ; XOR C
DI ; LD (HL),A
LD E,4 ; INC HL
LOOP DEFB #DD ; LD A,E
INC L ; ADD A,32
LD A,E ; AND #7F
DEFB #DD ; LD E,A
XOR H ; LD A,B
XOR (HL) ; OR C
DEFB #DD ; JR NZ,LOOP
DEC H ; RETAnd you can now disassemble the following of the loader... Simple, eh ? To finish with the non-official Z80 instructions, I would say that you can always order MMPF #5 to have more precisions... We're going on the assembler serial with the second part of the arithmetical operations, which is (excuse me, but the rest is in French)... CB 30 CB 31 CB 32 CB 33 CB 34 CB 35 CB 36 CB 37
SLI B SLI C SLI D SLI E SLI H SLI L SLI (HL) SLI A
DD CB 36 dis FD CB 36 dis et n'oubliez pas que 'dis'
SLI (IX+dis) SLI (IY+dis) est un nombre signé...Antoine / POW pour MM&PF| ★ AMSTRAD CPC ★ A voir aussi sur CPCrulez , les sujets suivants pourront vous intéresser... |
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