Computer Organization & Assembly Languages Integer Arithmetic

82 Pages·2008·0.34 MB·English

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Integer Arithmetic
Pu-Jen Cheng
Adapted from the slides prepared by Kip Irvine for the book,
Assembly Language for Intel-Based Computers, 5th Ed.
Chapter Overview
Shift and Rotate Instructions
(cid:132)
Shift and Rotate Applications
(cid:132)
Multiplication and Division Instructions
(cid:132)
Extended Addition and Subtraction
(cid:132)
ASCII and Unpacked Decimal Arithmetic
(cid:132)
Packed Decimal Arithmetic
(cid:132)
Shift and Rotate Instructions
Logical vs Arithmetic Shifts
(cid:132)
SHL Instruction
(cid:132)
SHR Instruction
(cid:132)
SAL and SAR Instructions
(cid:132)
ROL Instruction
(cid:132)
ROR Instruction
(cid:132)
RCL and RCR Instructions
(cid:132)
SHLD/SHRD Instructions
(cid:132)
Logical vs Arithmetic Shifts
A logical shift fills the newly created bit position
(cid:132)
with zero:
0
CF
• AAn ariitthhmettiic shhiifftt ffiilllls tthhe newlly creattedd bbiitt
position with a copy of the number’s sign bit:
CF
SHL Instruction
The SHL (shift left) instruction performs a logical
(cid:132)
left shift on the destination operand, filling the
lowest bit with 0.
0
CF
• Operand types for SHL: SHL destination,count
SHL reg,imm8
(Same for all shift and
SHL mem,imm8
rotate instructions)
SHL reg,CL
SHL mem,CL
Fast Multiplication
Shifting left 1 bit multiplies a number by 2
mov dl,5 Before: 0 0 0 0 0 1 0 1 = 5
shl dl,1
After: 0 0 0 0 1 0 1 0 = 10
SShiffting lefft n bits multiplies the operand by 2n
For example, 5 * 22 = 20
mov dl,5
shl dl,2 ; DL = 20
SHR Instruction
The SHR (shift right) instruction performs a logical
(cid:132)
right shift on the destination operand. The highest
bit position is filled with a zero.
0
CF
Shifting right n bits divides the operand by 2n
mov dl,80
shr dl,1 ; DL = 40
shr dl,2 ; DL = 10
SAL and SAR Instructions
SAL (shift arithmetic left) is identical to SHL.
(cid:132)
SAR (shift arithmetic right) performs a right
(cid:132)
arithmetic shift on the destination operand.
CF
An arithmetic shift preserves the number's sign.
mov dl,-80
sar dl,1 ; DL = -40
sar dl,2 ; DL = -10
Your turn . . .
Indicate the hexadecimal value of AL after each shift:
mov al,6Bh ; 01101011
shr al,1 a. 35h
shl al,3 b. A8h
mov al,8Ch ; 10001100
ssaarr aall,,11 cc.. CC66hh
sar al,3 d. F8h
ROL Instruction
ROL (rotate) shifts each bit to the left
(cid:132)
The highest bit is copied into both the Carry flag
(cid:132)
and into the lowest bit
No bits are lost
(cid:132)
CF
mov al,11110000b
rol al,1 ; AL = 11100001b
mov dl,3Fh
rol dl,4 ; DL = F3h