Program logic for crackers
redCourtesy of Reverser's page of reverse engineering
"Preparing +crackers for the next millennium"

(examples taken from PsP 41)
Inconditionals jumps

:00410310 EB10           jmp 00410322 ;Ebcb jump short
:004130E0 E92B010000     jmp 00413210 ;E9cw jump near
:00401D82 FF248DD8204000 jmp dword ptr [4*ecx + 004020D8]
                  ;jump near indirect
:0043133C EA114300F11143 jmp 4311:F1004311 
                  ;jump intersegment, 4-byte immediate
                  ;jump to call gate, ame privilege
                  ;jump via task state segment
                  ;jump via task gate

conditional jumps, special
	    E3CB	jcxz (rel8)	Jump short if CX (or ecx) = 0
:00423C64 E331    jcxz 00423C97

conditional short jumps, the '7' serie
          70nn   jo                (OF=1)
          71nnn  jno               (OF=0)
:00402FB3 7212   jb 00402FC7       (or jc or jnae) (CF=1)
          73nn   jnb               (or jnc or jae) (CF=0)
:0041321C 745B   je 00413279       (or jz)   (ZF=1)
:0041322C 7519   jne 00413247      (or jnz)  (ZF=0)
:0041323A 7604   jbe 00413240      (or jna)  (CF=1 or ZF=1)
:00410437 7702   ja 0041043B       (or jnbe) (CF=0 and ZF=0)
          78nn   js                (SF=1)
          79nn   jns               (SF=0)
          7Ann   jpe               (or jp)
          7Bnn   jpo               (or jnp)
          7Cnn   jl                (or jnge) (SF <> OF)
:00413450 7D2E   jge 00413480      (or jnl)  (SF = OF)
:00413619 7E07   jle 00413622      (or jng)  (ZF=1 and SF <> OF)
:0040435E 7F0C   jg 0040436C       (or jnle) (ZF=0 and SF = 0F)

conditional near jumps, the '0F' serie

:0041030A 0F8038FFFFFF  jo 004103D8     (OF=1)
:0040FFF2 0F81BA070000  jno 004107B2    (OF=0)
:004100C0 0F8295000000  jb 0041015B (or jc or jnae) (CF=1)
:004100CA 0F838B000000  jae 0041015B (or jnc or jnb) (CF=0)
:0041000C 0F84A0070000  jz 004107B2 (or je) (ZF=1)
:0041030A 0F8538FFFFFF  jnz 004103D8 (or jne) (ZF=0)
:0041030A 0F8638FFFFFF  jna 004103D8 (or jbe)(ZF=1 or CF=1)
:0040FFF2 0F87BA070000  ja 004107B2 (ZF=0 and CF=0)
:0041030A 0F8838FFFFFF  js 004103D8(SF=1)
:0041030A 0F8938FFFFFF  jns 004103D8 (SF=0)
:0041030A 0F8A38FFFFFF  jp 004103D8 (PF=1)
:0041030A 0F8B38FFFFFF  jnp 004103D8 (PF=0)
:004100C0 0F8C95000000  jl 0041015B (SF <> OF)
:004100CA 0F8D8B000000  jnl 0041015B (SF=OF)
:0041000C 0F8EA0070000  jle 004107B2 (or jng)(ZF=1 or SF <>OF)
:0041001E 0F8F55010000  jg 00410179 (ZF=0 and SF=OF)

Conditional jumps (except JCZX/JECXZ) test the flags which have 
been set by a previous instruction. The conditions for each mnemonic 
are given in parentheses after each description above. The terms 
"less" and "greater" are used for comparison of signed integers;
"above" and "below" are used for unsigned integers.

If the given condition is true, a jump is made to the location 
provided as the operand. instruction coding is most efficient
when the target for the conditional jump is in the current code
segment and within -128 to +127 bytes of the next instruction's 
first byte. The jump can also target -32768 through +32767 
(segment size attribute 16) or -2 to the 31st power to +2 to the
31st power -1 (segment size attribute 32) relative to the next 
instruction's first byte. 

When the target of the jump is in another segment, the
protectionist uses the opposite case of the jump instruction
(i.e. JE and JNE) and then access the target with an
unconditional far jump to the other segment.
JZ FarAway    will be therefore coded        JNZ BeyondFar
                                             JMP FarAway

JUMP BELOW OR EQUAL with cx comparison
:00413236 6683F908                cmp cx, 0008
:0041323A 7604                    jbe 00413240
:0041323C 33C0                    xor eax, eax
:0041323E EB0A                    jmp 0041324A
:00413240 B801000000              mov eax, 00000001

JUMP GREATER with ax testing
:00413445 6A10                    push 00000010

* Reference To: GetKeyState, Ord:00F9h in USER32.dll
:00413447 FF153C024C00            call dword ptr [004C023C]
:0041344D 6685C0                  test ax, ax
:00413450 7D2E                    jge 00413480

JUMP GREATER with ax/dx comparison
:00413814 668B90F4000000          mov dx, [eax-000000F4]
:0041381B 8D4301                  lea eax, [ebx+01]
:0041381E 3BD0                    cmp edx, eax
:00413820 7F14                    jg 00413836

:00413616 6685C9                  test cx, cx
:00413619 7E07                    jle 00413622

JUMP ABOVE after comparison ax/si
:00402007 663BF0                  cmp si, ax
:0040200A 668BF8                  mov di, ax
:0040200D 772A                    ja 00402039

Remember that for unsigned data (where all bits are treated as
data bits) a jump is equal, above or below, and for signed data
(where the leftmost bit is a sign) a jump is equal, grater or
less. JAE and JGE, although apparently similar, do not test the
same flags. The jumps for testing the carry, overflow and parity
flags have unique purposes.

Used after a test of unsigned data. If the CF flag is 0 (no
carry) and the ZF flag is zero (nonzero) it adds the operand
offset to the IP and performs the jump.

77             01110111

Used after a test of unsigned data. If the CF flag is 0 (no
carry) it adds the operand offset to the IP and performs the

73             01110011

The SCAS (scan string), CMPS (compare string) and LOOP
instructions use ZF to signale that their operations are
CF (Carry Flag)     Contains carries (0 or 1) from the high order
bit following arithmetic operations and some shift and rotate

PF (Parity Flag)    A check of the low-oder eight bits of data
operations. An odd number of data bits sets the flag to zero and
an even number to 1. Seldom used for protection schemes (seldom
used for conventional programming). Do not confond with the
parity bit!

AF (Auxiliary Carry Flag)     Set to 1 if arithmetic causes a
carry out of bit 3 (fourth from the right) of a register one-byte
operation. This flag is concerned wity arithmetic on ASCII and
packed decimal fields.

ZF (Zero Flag) Set as a result of arithmetic or compare
operations. Unexpectedely, a nonzero result sets it to ZERO and
a zero result set it to 1 wich is not apparently correct but
logically correct: zero means NO (THE RESULT IS NOT ZERO) and one
means YES (THE RESULT IS ZERO). JE and JZ test this flag.

SF (Sign Flag) Set according to the sign (high order or leftmost
bit) after an arithmetic operation: positive sets to zero and
negative sets to 1. JG and JL test this flag.

TF (Trap Flag) You have already set this flag entering the t
command in debug. When set, the trap flag causes the processor
to execute in single step mode, that is, one instruction at a
time under cracker control.

IF (Interrupt Flag) When 0 all interrupts are disabled, and when
1 all interrupts are enabled.

DF (Direction Flag) Used by string operations to determine the
direction of data transfer. When 0 the operation increments the
SI and DI registers, causing left to roght data transfer; when
1 the operation decrements the SI and DI causing right to left
data transfer.

OF (Overflow Flag)  Indicates a carry into and out of the high
order (leftmost) sign bit following a signed arithmetic
The flags register in the 80386 and upwards is a 32 bit register
named EFLAGS. We have status flags, control flags and system
flags. The low order 16 bits of EFLAGS is called FLAGS and can
be trated as an unit, because is identical to the FLAGS register
of the 8086 and 80286.

The CMP instructions 3Cib CMP AL,imm8 Compare immediate byte to AL 3Diw CMP AX,imm16 Compare immediate word from AX 3Did CMP EAX,imm32 Compare immediate dword to EAX 80 /7 ib CMP r/m8,imm8 Compare immediate byte to r/m byte 81 /7 iw CMP r/m16,imm16 Compare immediate word to r/m word 81 /7 id CMP r/m32,imm32 Compare immediate dword to r/m dword 83 /7 ib CMP r/m16,imm8 Compare sign extended immediate byte to r/m word 83 /7 ib CMP r/m32,imm8 Compare sign extended immediate byte to r/m dword CMP subtracts the secodn operand from the first, but, unlike the SUB instruction, does not store the result; only the flags are changed. If an operand greater than one byte is compared to an immediate byte, the byte value is first sign-extended.

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reverser 27 May 97