AMD Am186 Series, Instruction Set

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Instruction Set 4-217
SBB SBB
Flag Settings After Instruction
Examples
This example subtracts one 64-bit unsigned number in a register (the subtrahend) from
another 64-bit unsigned number in memory (the minuend).
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Processor Status
Flags Register
reserved
OF DF IF TF SF ZF AF PF CF
? = undefined; – = unchanged
OF=1 if result larger than destination operand
OF=0 otherwise
AF=1 if carry or borrow to low nibble
AF=0 otherwise
PF=1 if low byte of result has even number of set bits
PF=0 otherwise
CF=1 for carry or borrow to high-order bit
CF=0 otherwise
– – – res res res
SF=1 if result is 0 or positive
SF=0 if result is negative
ZF=1 if result equal to 0
ZF=0 if result not equal to 0
UMINUEND DQ 3B865520F4DE89A1h
USUBTRAHEND DQ 0C285DE70893BB2Ah
WSIZE EQU 2
QSIZE EQU 8
; 64-bit unsigned subtraction: UMINUEND = UMINUEND - USUBTRAHEND
; left (low) word subtraction
MOV AX,WORD PTR USUBTRAHEND ; copy subtrahend
SUB WORD PTR UMINUEND,AX ; subtract
; set up bases and index
MOV SI,WORD PTR UMINUEND ; minuend base
MOV DI,WORD PTR USUBTRAHEND ; subtrahend base
MOV BX,WSIZE ; set up index
NEXT:
; next higher word subtraction
MOV AX,[BX][DI] ; copy subtrahend
SBB [BX][SI],AX ; subtract with borrow
; increase index and compare
ADD BX,WSIZE ; point to next word
CMP BX,QSIZE ; is this the last word?
; if not last word, then jump to top of loop
JNE NEXT