turbo-pascal-assembly

Random Function: (Real), FPU: 87/287/387

Returns a random number (Real type, 6-bytes) in the range 0.0 <= X < 1.0. It is the same as the Random: Real function but uses FPU code. The main program is compiled with 8087 emulation disabled (-$E-) and 8087/80287/80387 code enabled (-$N+):

TPC.EXE -GS -GP -GD -$D+ -$E- -$N+ MAIN

Random: Real using FPU

CODE0019: CALL SYS:0D27
CODE001E: CALL SYS:098E

Random(Real) is composed of two separate calls to System Library subroutines (SYS:0D27) and (SYS:098E). The results (Real, 6 bytes) is returned in DX:BX:AX where:

• DX = High Word
• BX = Middle Word
• AX = Low Word

Random: Real using FPU (I)

SYS0D27: CALL 0D4A

Call random number generator engine at SYS:0D4A.

SYS0D2A: FILD CS:WORD PTR [0D48]
SYS0D30: FILD DWORD PTR [RandSeed]
SYS0D35: FADD CS:DWORD PTR [0D44]
SYS0D3A: FSCALE
SYS0D3E: FSTP ST(1)
SYS0D41: FWAIT
SYS0D43: RETF

DWORD in SYS:0D44

SYS0D44:  00 00 00 4F

WORD in SYS:0D48

SYS0D48:  E0 FF

SYS:0D4A Random Number Generator Engine

This is the same random number generator engine (see: SYS:05DD Random Number Generator Engine for analysis). The location of this routine has been displaced due to inclusion of additional routines in the System Library.

SYS0D4A: MOV AX,[RandSeed.Low]
SYS0D4D: MOV BX,[RandSeed.High]
SYS0D51: MOV CX,AX
SYS0D53: CS:
SYS0D54: MUL WORD PTR [0D80]
SYS0D58: SHL CX,1
SYS0D5A: SHL CX,1
SYS0D5C: SHL CX,1
SYS0D5E: ADD CH,CL
SYS0D60: ADD DX,CX
SYS0D62: ADD DX,BX
SYS0D64: SHL BX,1
SYS0D66: SHL BX,1
SYS0D68: ADD DX,BX
SYS0D6A: ADD DH,BL
SYS0D6C: MOV CL,05
SYS0D6E: SHL BX,CL
SYS0D70: ADD DH,BL
SYS0D72: ADD AX,0001
SYS0D75: ADC DX,+00
SYS0D78: MOV [RandSeed.Low],AX
SYS0D7B: MOV [RandSeed.High],DX
SYS0D7F: RET

WORD in SYS:0D80

SYS0D80:  05 84

Magic number used bye the random number generator engine (8405h/33797).

Random: Real using FPU (II)

SYS098E: SUB SP,+0A

Reserve 10 bytes on the stack.

SYS0991: MOV BX,SP

Copy offset to this location (SP) into BX.

SYS0993: FSTP SS:TBYTE PTR [BX]
SYS0997: FWAIT

Load 10 bytes from FPU stack into reserved location [BX].

SYS0999: ADD SP,+02

Ignore MSB

SYS099C: POP CX
SYS099D: POP BX
SYS099E: POP DX
SYS099F: POP AX

Load into AX:DX:BX:CX.

SYS09A0: MOV DI,AX
SYS09A2: AND AX,7FFF
SYS09A5: SUB AX,3F7E
SYS09A8: JBE 09C4
SYS09AA: OR AH,AH
SYS09AC: JNZ 09CF
SYS09AE: MOV AH,CH
SYS09B0: SHL CL,1
SYS09B2: ADC AH,00
SYS09B5: ADC BX,+00
SYS09B8: ADC DX,+00
SYS09BB: JB 09CB
SYS09BD: SHL DX,1
SYS09BF: SHL DI,1
SYS09C1: RCR DX,1
SYS09C3: RETF
SYS09C4: XOR AX,AX
SYS09C6: XOR BX,BX
SYS09C8: XOR DX,DX
SYS09CA: RETF
SYS09CB: INC AL
SYS09CD: JNZ 09BD
SYS09CF: MOV AX,00CD
SYS09D2: JMP 010F

See also: RandSeed, SYS:05DD Random Number Generator Engine or go back

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