AAM

ASCII Adjust AX After Multiply

stableVMJITAOTinstruction

Encodings

Measured cost

Not measured: it does not run on 64-bit processors.

Flags named

• CF Carry
• PF Parity
• AF Auxiliary carry
• ZF Zero
• SF Sign
• OF Overflow

Description#

Adjusts the result of the multiplication of two unpacked BCD values to create a pair of unpacked (base 10) BCD values. The AX register is the implied source and destination operand for this instruction. The AAM instruction is only useful when it follows an MUL instruction that multiplies (binary multiplication) two unpacked BCD values and stores a word result in the AX register. The AAM instruction then adjusts the contents of the AX register to contain the correct 2-digit unpacked (base 10) BCD result.

The generalized version of this instruction allows adjustment of the contents of the AX to create two unpacked digits of any number base (see the "Operation" section below). Here, the imm8 byte is set to the selected number base (for example, 08H for octal, 0AH for decimal, or 0CH for base 12 numbers). The AAM mnemonic is interpreted by all assemblers to mean adjust to ASCII (base 10) values. To adjust to values in another number base, the instruction must be hand coded in machine code (D4 imm8).

This instruction executes as described in compatibility mode and legacy mode. It is not valid in 64-bit mode.

Operation#

IF 64-Bit Mode
    THEN
          #UD;
    ELSE
          tempAL := AL;
          AH := tempAL / imm8; (* imm8 is set to 0AH for the AAM mnemonic *)
          AL := tempAL MOD imm8;

FI;

The immediate value (imm8) is taken from the second byte of the instruction.

Flags affected#

The SF, ZF, and PF flags are set according to the resulting binary value in the AL register. The OF, AF, and CF flags are undefined.

Exceptions

Sources

• This instruction in the Intel manual