Category : Assembly Language Source Code
Archive   : MATHASM.ZIP
Filename : MUL_F.ASM

MUL_F PROC NEAR
;***************************************************************
; floating point
; multiplication routine
;***************************************************************
PUSH SI ;save index registers
PUSH DI
SUB DX,DX ;clear work registers
MOV AX,DX
MOV BX,DX
MOV CX,DX
; check for zero operand
OR DX,[DI]+2 ;OP1 high word in DX
JZ MF2 ;zero only if operand is 0
OR CX,[SI]+2 ;OP2 high word in CX
JZ MF2
; compute exponent
MOV AL,DH ;E1 in AX
MOV BL,CH ;E2 in BX
SUB BX,128 ;subtract bias
ADD AX,BX ;E=E1+E2+bias
;
MOV BX,[SI] ;get OP2 low word in BX
MOV SI,AX ;save exponent in SI
MOV AX,[DI] ;get OP1 low word in AX
SUB DH,DH ;clear high bytes
MOV CH,DH ;M1 is DL:AX; M2 is CL:BX
; compute result sign
MOV DI,DX
XOR DI,CX
AND DI,10000000B ;isolate sign bit in DI
; restore leading ones
OR DL,10000000B
OR CL,10000000B
; multiply mantissas
CALL MUL32 ;47-48 bit product in AX:CX:BX
;
OR AX,AX ;is it normalized?
JS MF1
; normalize product
SHL BX,1 ;shift it left 1
RCL CX,1
RCL AX,1
SUB SI,1 ;and decrement exponent
; check for overflow
MF1: CMP SI,255
JG MF3 ;exit if exponent > 255
; check for underflow
CMP SI,0
JLE MF4 ;exit if exponent <= 0
; reformat for output
MOV DL,AH ;mantissa in DL:AX
MOV AH,AL
MOV AL,CH
MOV DH,CL ;trailing bits in DH
MOV CX,BX ;and CX
MOV BX,SI ;exponent in BH
MOV BH,BL
JMP R0 ;ROUND in addition routine
MF2: JMP FINISH
MF3: JMP OVER_F
MF4: JMP UNDER_F
MUL_F ENDP