das

Decimal Adjust After Subtraction

DAS

Adjusts AL after subtraction for packed BCD.

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);

Encoding

Binary Layout
2F
+0
 
Format Legacy
Opcode 2F
Extension Base (Legacy)

Operands

Related

More in Base (Legacy)

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
2F DAS ZO Invalid Valid Decimal adjust AL after subtraction.

Description

Adjusts the result of the subtraction of two packed BCD values to create a packed BCD result. The AL register is the implied source and destination operand. The DAS instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction) one 2-digit, packed BCD value from another and stores a byte result in the AL register. The DAS instruction then adjusts the contents of the AL register to contain the correct 2-digit, packed BCD result. If a decimal borrow 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 (Borrow from AL := AL - 6);
AF := 1;
ELSE
AF := 0;
FI;
IF ((old_AL > 99H) or (old_CF = 1))
THEN
AL := AL - 60H;
CF := 1;
FI;
FI;

Example
SUB    AL, BL   Before: AL = 35H, BL = 47H, EFLAGS(OSZAPC) = XXXXXX
After: AL = EEH, BL = 47H, EFLAGS(0SZAPC) = 010111
DAA             Before: AL = EEH, BL = 47H, EFLAGS(OSZAPC) = 010111
After: AL = 88H, BL = 47H, EFLAGS(0SZAPC) = X10111

Flags Affected

The CF and AF flags are set if the adjustment of the value results in a decimal borrow 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. DAS-Decimal Adjust AL After Subtraction Vol. 2A 3-260

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. DAS-Decimal Adjust AL After Subtraction Vol. 2A 3-261