AMD Am186 Series, Instruction Set, Page: 256 / 320

Instruction Set 4-216
SBB Subtract Numbers with Borrow SBB
What It Does
SBB subtracts an integer or an unsigned number and the value of the Carry Flag (CF) from
another number of the same type.
Syntax
Description
SBB adds the second operand (
subtrahend) to CF and subtracts the result from the first
operand ( difference). The result of the subtraction is assigned to the first operand and the
flags are set accordingly.
Operation It Performs
Form Opcode Description
Clocks
Am186 Am188
SBB AL,
imm8
1C
ib
Subtract immediate byte from AL with borrow 3 3
SBB AX, imm16 1D iw Subtract immediate word from AX with borrow 4 4
SBB r/m8,imm8 80 /3 ib Subtract immediate byte from r/m byte with borrow 4/16 4/16
SBB r/m16,imm16 81 /3 iw Subtract immediate word from r/m word with borrow 4/16 4/20
SBB r/m16,imm8 83 /3 ib Subtract sign-extended imm. byte from r/m word with borrow 4/16 4/20
SBB r/m8,r8 18 /r Subtract byte register from r/m byte with borrow 3/10 3/10
SBB
r/m16,r16
19
/r
Subtract word register from r/m word with borrow 3/10 3/14
SBB
r8,r/m8
1A
/r
Subtract r/m byte from byte register with borrow 3/10 3/10
SBB r16,r/m16 1B /r Subtract r/m word from word register with borrow 3/10 3/14
SBB
difference,subtrahend
if (size( difference) == 16)
if (size( subtrahend) == 8)
/* extend sign of subtrahend */
if ( subtrahend < 0)
subtrahend = 0xFF00 | subtrahend;
else
subtrahend = 0x00FF & subtrahend;
/* subtract with borrow */
difference = difference - subtrahend - CF;