Intel 80286 BINARY-CODED Decimal Arithmetic Instructions, Packed BCD Adjustment Instructions

BASIC INSTRUCTION SET

15 14 13 12 11 10 9 8 7 6 5 4 3 2

STACK WORD

PUSHF decrements SP by 2 bytes (1 word) and copies the contents of Ihe flag reglsler 10 Ihe lop of slack. POPF loads Ihe flag reg Isler wllh Ihe conlenls of Ihe lasl word pushed onlo Ihe stack. The bit position of each flag Is the same In the stack word as It Is In Ihe flag register. Only programs executing allhe hlghesl privilege level (level 0) may alter the 2-blt 10PL flag. Only programs executing al a level at leasl as privileged as Ihallndlcated by 10PL may alter IF.

G30108

Figure 3-14. PUSHF and POPF

Procedures may use this instruction to restore the flag status from a previous value.

Example: POPF

3.10 BINARY-CODED DECIMAL ARITHMETIC INSTRUCTIONS

These instructions adjust the results of a previous arithmetic operation to produce a valid packed or unpacked decimal result. These instructions operate only on AL or AH registers.

3.10.1 Packed BCD Adjustment Instructions

DAA (Decimal Adjust) corrects the result of adding two valid packed decimal operands in AL. DAA must always follow the addition of two pairs of packed decimal numbers (one digit in each nibble) to obtain a pair of valid packed decimal digits as results. The carry flag will be set if carry was needed.

Example: DAA

DAS (Decimal Adjust for Subtraction) corrects the result of subtracting two valid packed decimal operands in AL. DAS must always follow the subtraction of one pair of packed decimal numbers (one digit in each nibble) from another to obtain a pair of valid packed decimal digits as results. The carry flag will be set if a borrow was needed.

Example: DAS

3.10.2 Unpacked BCD Adjustment Instructions

AAA(ASCII Adjust for Addition) changes the contents of register AL to a valid unpacked decimal number, and zeros the top 4 bits. AAA must always follow the addition of two unpacked decimal operands in AL. The carry flag will be set and AH will be incremented if a carry was necessary.

Example: AAA

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