daa

Decimal Adjust After Addition

DAA

Adjusts AL after addition for packed BCD.

Details

Adjusts AL after addition of two packed BCD values. If the lower nibble exceeds 9 or AF is set, adds 6 to AL; if the upper nibble then exceeds 9 or CF is set, adds 0x60 to AL. Sets CF and AF as needed; OF is undefined; SF, ZF, PF are computed on the result.

Pseudocode Operation

old_AL ← AL;
if ((AL & 0x0F) > 9) || (AF == 1) {
  AL ← AL + 6;
  AF ← 1;
} else {
  AF ← 0;
}
if (old_AL > 0x99) || (CF == 1) {
  AL ← AL + 0x60;
  CF ← 1;
} else {
  CF ← 0;
}
ZF ← (AL == 0);
SF ← (AL[7] == 1);
PF ← parity(AL);

Example

DAA

Encoding

Binary Layout
27
+0
 
Format Legacy
Opcode 27
Extension Base (Legacy)

Operands

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
27 DAA ZO Invalid Valid Decimal adjust AL after addition.

Description

Adjusts the sum of two packed BCD values to create a packed BCD result. The AL register is the implied source and destination operand. The DAA instruction is only useful when it follows an ADD instruction that adds (binary addition) two 2-digit, packed BCD values and stores a byte result in the AL register. The DAA instruction then adjusts the contents of the AL register to contain the correct 2-digit, packed BCD result. If a decimal carry is detected, the CF and AF flags are set accordingly. This instruction executes as described above in compatibility mode and legacy mode. It is not valid in 64-bit mode.

Operation

IF 64-Bit Mode
THEN
#UD;
ELSE
old_AL := AL;
old_CF := CF;
CF := 0;
IF (((AL AND 0FH) > 9) or AF = 1)
THEN
AL := AL + 6;
CF := old_CF or (Carry from AL := AL + 6);
AF := 1;
ELSE
AF := 0;
FI;
IF ((old_AL > 99H) or (old_CF = 1))
THEN
AL := AL + 60H;
CF := 1;
ELSE
CF := 0;
FI;
FI;

Example
ADD    AL, BL   Before: AL=79H BL=35H EFLAGS(OSZAPC)=XXXXXX
After: AL=AEH BL=35H EFLAGS(0SZAPC)=110000
DAA             Before: AL=AEH BL=35H EFLAGS(OSZAPC)=110000
After: AL=14H BL=35H EFLAGS(0SZAPC)=X00111
DAA             Before: AL=2EH BL=35H EFLAGS(OSZAPC)=110000
After: AL=34H BL=35H EFLAGS(0SZAPC)=X00101





DAA—Decimal Adjust AL After Addition                                                                                                          Vol. 2A 3-258

Flags Affected

The CF and AF flags are set if the adjustment of the value results in a decimal carry in either digit of the result (see the “Operation” section above). The SF, ZF, and PF flags are set according to the result. The OF flag is undefined.

Exceptions

Protected Mode Exceptions

#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#UD If the LOCK prefix is used.

Compatibility Mode Exceptions

#UD If the LOCK prefix is used.

64-Bit Mode Exceptions

#UD If in 64-bit mode. DAA—Decimal Adjust AL After Addition Vol. 2A 3-259