Notes on the Instruction Set Reference

Instruction Operands

When an instruction takes two operands, the destination operand is the one on the left, and the source operand is the one on the right. In general, when a result is produced by an instruction, the result replaces the destination operand. For example, in this instruction:

ADD BX,SI

the BX register is added to the SI register, and the sum is then placed in the BX register, overwriting whatever was in BX before the addition.

Flag Results

Each instruction contains a flag summary that looks like this (the asterisks will vary from instruction to instruction):

O D I T S Z A P C  OF: Overflow flag  TF: Trap flag AF: Aux carry
F F F F F F F F F  DF: Direction flag SF: Sign flag PF: Parity flag
*       * * * * *  IF: Interrupt flag ZF: Zero flag CF: Carry flag

The nine flags are all represented here. An asterisk indicates that the instruction on that page affects that flag. If a flag is affected at all (that is, if it has an asterisk beneath it), it will be affected according to these rules:

OF	Set if the result is too large to fit in the destination operand.
IF	Set by the STI instruction; cleared by CLI.
TF	For debuggers; not used in normal programming and may be ignored.
SF	Set when the sign of the result forces the destination operand to become negative.
ZF	Set if the result of an operation is zero. If the result is nonzero, ZF is cleared.
AF	Auxiliary carry used for 4-bit BCD math. Set when an operation causes a carry out of a 4-bit BCD quantity.
PF	Set if the number of 1 bits in the low byte of the result is even; cleared if the number of 1 bits in the low byte of the result is odd. Used in data communications applications but little else.
CF	Set if the result of an add or shift operation carries out a bit beyond the destination operand; otherwise cleared. May be manually set by STC and manually cleared by CLC when CF must be in a known state before an operation begins.

Some instructions force certain flags to become undefined. When this is the case, it is noted under "Notes." Undefined means don't count on it being in any particular state.