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

RN_F PROC NEAR
;********************************************************
; Rounds a floating point operand to
; the nearest integer value
;********************************************************
PUSH SI ;save index registers
PUSH DI

MOV BX,CX ;and BX
MOV DX,[DI]+2 ;get operand in DX:AX
MOV AX,[DI]
;
SUB CX,CX ;clear CX
MOV DI,10000000B ;isolate sign in DI
AND DI,DX
OR DX,10000000B ;restore leading one
; check range
CMP DH,128 ;exponent bias is 128
JA RN1
JB RN7 ;if exponent<0 round to zero
; exponent is zero
CMP DL,128 ;number >= .5 and < 1
JBE RN7 ;if fraction=0 round to zero
MOV DX,CX ;round to +1-all zeros
MOV AX,CX
MOV BH,81H ;and biased exponent of 1
JMP RN6 ;restore sign and exit
;
RN1: MOV BH,DH ;exponent in BH in case we exit
CMP DH,152 ;if exponent>=24, already integer
JAE RN6 ;so restore sign and exit
; exponent is in range 1-23
MOV CL,DH ;put unbiased exponent in CX
SUB CX,128
NEG CX ;COUNT is CX - 24
ADD CX,24
; shift right COUNT bits
SUB DH,DH ;clear DH
MOV BH,DH ;and BH
RN2: SHR DL,1 ;shift it right
RCR AX,1
RCR DH,1 ;rotate fraction into DH:BX
RCR BX,1
LOOP RN2
; round
OR DH,DH ;is guard bit on?
JNS RN4 ;if not, rounding not needed
TEST AX,1 ;if integer odd
JNZ RN3 ;round up
CMP DH,80H ;if any trailing bits on
JA RN3 ;round up
OR BX,BX
JZ RN4
RN3: ADD AX,1 ;round up
ADC DX,0
; normalize--shift left
RN4: MOV BH,152 ;set exponent to zero
RN5: SHL AX,1 ;shift left
RCL DL,1
DEC BH ;increment exponent
OR DL,DL
JNS RN5 ;shift again if not normalized
;
RN6: JMP FINISH ;restore sign and exit
RN7: SUB DX,DX ;zero result
MOV AX,DX
MOV BX,DX
JMP EXIT
RN_F ENDP