Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1390

DAS—Decimal Adjust AL after Subtraction
Opcode
2F
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.
Operation
IF (AL AND 0FH) > 9 OR AF = 1
THEN
AL  AL  6;
CF CF OR BorrowFromLastSubtraction; (* CF OR borrow from AL AL  6 *)
AF  1;
ELSE AF  0;
FI;
IF ((AL > 9FH) or CF = 1)
THEN
AL  AL  60H;
CF  1;
ELSE CF  0;
FI;
Example
SUB AL, BL
DAA
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 "Operation" above). The SF, ZF, and PF flags are set
according to the result. The OF flag is undefined.
Additional Itanium System Environment Exceptions
Itanium Reg Faults NaT Register Consumption Abort.
Exceptions (All Operating Modes)
None.
4:88
Instruction Description
DAS Decimal adjust AL after subtraction
Before: AL=35H BL=47H EFLAGS(OSZAPC)=XXXXXX
After: AL=EEH BL=47H EFLAGS(0SZAPC)=010111
Before: AL=EEH BL=47H EFLAGS(OSZAPC)=010111
After: AL=88H BL=47H EFLAGS(0SZAPC)=X10111
Volume 4: Base IA-32 Instruction Reference