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This is CM3 by:

: ${L:-aura} #

: #

DB 60           ;PUSHA      all registers saved. when the decoder has finished
                ;           it jumps to the created data which will begin with
                ;           the relocation code. The relocation code when done
                ;           will issue a POPA to get all registers (minus FLAGS)
                ;           back to as they were at the start of the DOS .COM
                ;           then JMP 0x0100
DB 68 7D 61     ;PUSH 617D
DB 58           ;POP AX
DB 50           ;PUSH AX
DB 35 79 60     ;XOR AX,6079         
DB 50           ;PUSH AX
DB 5D           ;POP BP     BP=104  this value is kept in BP throughout
DB 34 6E        ;XOR AL,6E           
DB 50           ;PUSH AX
DB 5F           ;POP DI     DI=16A =rstart, the end of the decoder body
                ;           and the first byte of encoded data
DB 58           ;POP AX     AX=617D is used to apply fixups below
DB 57           ;PUSH DI    stack 016A. this is popped immediately
                ;           on entry to the main loop into the SI register
                ;           due to POP SI being 0x5E (a disallowed code)
                ;           the instruction is created in the loop body
                ;           at fixup *F7*
; apply fixups to decoder body, all these bytes could not
; be represented within the valid code set so have to be adjusted
; note the offsets are relative to 0104 in BP
SUB [BP+34],AL  ;*F1*
SUB [BP+36],AL  ;*F2*
SUB [BP+3D],AL  ;*F3*
SUB [BP+46],AL  ;*F4*
SUB [BP+48],AX  ;*F5*
SUB [BP+4C],AL  ;*F6*
SUB [BP+65],AL  ;*F8*
SUB [BP+52],AL  ;*F9*
XOR [BP+54],AL  ;*F7*
DB 73 2A        ;JNC K0      CF=0 due to XOR above so this branches always
                ;            Apart from jumping to the main loop this branch
                ;            also serves the purpose of forcing a flush of
                ;            the prefetch queue so that all modified bytes
                ;            will be seen by the processor. Pentium+ CPUs
                ;            flush the prefetch queue any bytes within it
                ;            are modified, automatically, however pre-Pentium
                ;            CPUs do not.
; table for translating ascii codes back to base-85 value
; 0D of the line break is also the last element of the table
;:TAB = 012E
DB 7D 60 41 3F 2B 2C
DB 0D 0A        ; CR,LF line break. end of 1st line of decoder ascii-code
; during the main loop registers are used-
; EDX=current dword data being made
;  SI=read data pointer
;  DI=write data pointer
;  BP=0104     (used to get base offset to access table :TAB)
; EAX=each byte read is converted to a base-85 value in EAX
;  CX=valid bytes left to read to complete building current dword in EDX
;:L2  = 0136
DB 66 6B 4F 55  ;IMUL EDX,EDX,55    fixup *F1* at 0138: 4F-7D=D2
;:L0  = 013A
DB 29           ;LODSB              fixup *F2* at 013A: 29-7D=AC
;:L1  = 013B
; convert ascii value in AL to base-85 value
; increment AL once plus once for each 'hole' it lies below
; this will translate ascii values so that we have a contiguous
; sequence starting at 0x2A, for ascii 0x23. 
DB 3A 47 2A     ;CMP AL,[BX+2A]     0104+2A=012E =TAB data table
DB 40           ;INC AX
DB 43           ;INC BX
;     = 0140
DB 72 76        ;JC L1              fixup *F3* at 0141: 76-7D=F9 (-07)
; de-biasing the value back to 0 based is done with a SUB and then
; a DEC so that the 0x23 ascii value can be picked up (by now it will
; be 0x2A) as well as having CF=1 for any values before 0x23
DB 2C 2A        ;SUB AL,2A
DB 74 24        ;JZ rstart     
DB 48           ;DEC AX
;:L3  = 0147
DB 55           ;PUSH BP     
DB 5B           ;POP BX             BX=0104
;     = 0149
; if the carry flag is set now from the SUB above the value was before
; the first code 0x24 (normally this will be 0x0D and 0x0A but in future
; will probably include 0x21 which may be used as a padding character)
; so ignore it.
DB 72 6C        ;JC L0              fixup *F4* at 014A: 6C-7D=EF (-17)
; next base-85 chunk added to total dword, continue looping
; if more.
DB 66 7E 23     ;ADD EDX,EAX        fixup *F5* at 014C: 237E-617D=C201
DB 49           ;DEC CX  
DB 75 62        ;JNZ  L2            fixup *F6* at 0150: 62-7D=E5 (-1B)
;     = 0151
; write next decoded dword to store
DB 66 52        ;PUSH EDX
DB 66 58        ;POP EAX
DB 66 28        ;STOSD              fixup *F9* at 0155: 28-7D=AB
DB 56           ;PUSH SI
; initial entry is here so that SI is initialised from the stack
; in subsequent loops the above PUSH cancels this instruction
;K0   = 0158
DB 23           ;POP SI             fixup *F7* at 0158: 23^7D=5E
; set EAX=EDX=00000000 ready to create next dword
; set CX=6, bytes to read to make the base-85 value
; this is actually 5 but we jump back to the loop via L3
; so there is an extra decrement of the count
DB 66 6A 7D     ;PUSHD 7D
DB 66 58        ;POP EAX       
DB 34 7B        ;XOR AL,7B          EAX=00000006
DB 50           ;PUSH AX  
DB 59           ;POP CX             CX=0006
DB 24 40        ;AND AL,40          EAX=00000000
DB 66 50        ;PUSH EAX
DB 66 5A        ;POP EDX            EDX=00000000
;     = 0168
; go back to loop at L0, but via L3 so that BX gets loaded properly
; on the very first loop. CF=0 from the AND instruction above so
; this branch always occurs.
DB 73 5A        ;JNC L3             fixup *F8* at 0169: 5A-7D=DD (-23)
;     = 016A
; encoded data starts here. when decoded the first byte will start here
; also so the decoder jumps here when finished (again the jump is
; important to ensure a prefetch queue flush)

file: /Techref/DOS/cm3_txt.htm, 5KB, , updated: 2004/5/20 16:31, local time: 2023/2/7 03:03,

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