Transcript Lecture Notes

Assembly Language for Intel-Based
Computers, 4th Edition
Kip R. Irvine
Chapter 7: Integer Arithmetic
Slides prepared by Kip R. Irvine
Revision date: 11/08/2002
• Chapter corrections (Web) Assembly language sources (Web)
(c) Pearson Education, 2002. All rights reserved. You may modify and copy this slide show for your personal use, or for
use in the classroom, as long as this copyright statement, the author's name, and the title are not changed.
Chapter Overview
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Shift and Rotate Instructions
Shift and Rotate Applications
Multiplication and Division Instructions
Extended Addition and Subtraction
ASCII and Packed Decimal Arithmetic
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
2
Shift and Rotate Instructions
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•
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Logical vs Arithmetic Shifts
SHL Instruction
SHR Instruction
SAL and SAR Instructions
ROL Instruction
ROR Instruction
RCL and RCR Instructions
SHLD/SHRD Instructions
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
3
Logical vs Arithmetic Shifts
• A logical shift fills the newly created bit position with
zero:
0
CF
• An arithmetic shift fills the newly created bit position
with a copy of the number’s sign bit:
CF
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
4
SHL Instruction
• The SHL (shift left) instruction performs a logical left
shift on the destination operand, filling the lowest bit
with 0.
0
CF
• Operand types for SHL:
SHL reg,imm8
SHL mem,imm8
SHL reg,CL
SHL mem,CL
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
(Same for all shift and
rotate instructions)
Web site
Examples
5
Fast Multiplication
Shifting left 1 bit multiplies a number by 2
mov dl,5
shl dl,1
Before:
00000101
=5
After:
00001010
= 10
Shifting left n bits multiplies the operand by 2n
For example, 5 * 22 = 20
mov dl,5
shl dl,2
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
; DL = 20
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Examples
6
SHR Instruction
• The SHR (shift right) instruction performs a logical
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
shr dl,2
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
; DL = 40
; DL = 10
Web site
Examples
7
SAL and SAR Instructions
• SAL (shift arithmetic left) is identical to SHL.
• SAR (shift arithmetic right) performs a right arithmetic
shift on the destination operand.
CF
An arithmetic shift preserves the number's sign.
mov dl,-80
sar dl,1
sar dl,2
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
; DL = -40
; DL = -10
Web site
Examples
8
Your turn . . .
Indicate the hexadecimal value of AL after each shift:
mov
shr
shl
mov
sar
sar
al,6Bh
al,1
al,3
al,8Ch
al,1
al,3
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
a. 35h
b. A8h
c. C6h
d. F8h
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Examples
9
ROL Instruction
• ROL (rotate) shifts each bit to the left
• The highest bit is copied into both the Carry flag
and into the lowest bit
• No bits are lost
CF
mov al,11110000b
rol al,1
; AL = 11100001b
mov dl,3Fh
rol dl,4
; DL = F3h
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
10
ROR Instruction
• ROR (rotate right) shifts each bit to the right
• The lowest bit is copied into both the Carry flag and
into the highest bit
• No bits are lost
CF
mov al,11110000b
ror al,1
; AL = 01111000b
mov dl,3Fh
ror dl,4
; DL = F3h
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
11
Your turn . . .
Indicate the hexadecimal value of AL after each rotation:
mov al,6Bh
ror al,1
rol al,3
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
a. B5h
b. ADh
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Examples
12
RCL Instruction
• RCL (rotate carry left) shifts each bit to the left
• Copies the Carry flag to the least significant bit
• Copies the most significant bit to the Carry flag
CF
clc
mov bl,88h
rcl bl,1
rcl bl,1
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
;
;
;
;
CF = 0
CF,BL = 0 10001000b
CF,BL = 1 00010000b
CF,BL = 0 00100001b
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Examples
13
RCR Instruction
• RCR (rotate carry right) shifts each bit to the right
• Copies the Carry flag to the most significant bit
• Copies the least significant bit to the Carry flag
CF
stc
mov ah,10h
rcr ah,1
; CF = 1
; CF,AH = 1 00010000b
; CF,AH = 0 10001000b
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
14
Your turn . . .
Indicate the hexadecimal value of AL after each rotation:
stc
mov al,6Bh
rcr al,1
rcl al,3
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
a. B5h
b. AEh
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Examples
15
SHLD Instruction
• Shifts a destination operand a given number of bits to
the left
• The bit positions opened up by the shift are filled by
the most significant bits of the source operand
• The source operand is not affected
• Syntax:
SHLD destination, source, count
• Operand types:
SHLD reg16/32, reg16/32, imm8/CL
SHLD mem16/32, reg16/32, imm8/CL
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
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SHLD Example
Shift wval 4 bits to the left and replace its lowest 4 bits with
the high 4 bits of AX:
.data
wval WORD 9BA6h
.code
mov ax,0AC36h
shld wval,ax,4
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
wval
AX
Before:
9BA6
AC36
After:
BA6A
AC36
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Examples
17
SHRD Instruction
• Shifts a destination operand a given number of bits to
the right
• The bit positions opened up by the shift are filled by
the least significant bits of the source operand
• The source operand is not affected
• Syntax:
SHRD destination, source, count
• Operand types:
SHRD reg16/32, reg16/32, imm8/CL
SHRD mem16/32, reg16/32, imm8/CL
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
18
SHRD Example
Shift AX 4 bits to the right and replace its highest 4 bits with
the low 4 bits of DX:
mov ax,234Bh
mov dx,7654h
shrd ax,dx,4
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
DX
AX
Before:
7654
234B
After:
7654
4234
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Examples
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Your turn . . .
Indicate the hexadecimal values of each destination
operand:
mov
mov
shld
shrd
ax,7C36h
dx,9FA6h
dx,ax,4
dx,ax,8
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
; DX = FA67h
; DX = 36FAh
Web site
Examples
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Shift and Rotate Applications
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•
•
•
Shifting Multiple Doublewords
Binary Multiplication
Displaying Binary Bits
Isolating a Bit String
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
Web site
Examples
21
Shifting Multiple Doublewords
• Programs sometimes need to shift all bits within an
array, as one might when moving a bitmapped
graphic image from one screen location to another.
• The following shifts an array of 3 doublewords 1 bit to
the right (view complete source code):
.data
ArraySize = 3
array DWORD ArraySize DUP(99999999h)
; 1001 1001...
.code
mov esi,0
shr array[esi + 8],1
; high dword
rcr array[esi + 4],1
; middle dword, include Carry
rcr array[esi],1
; low dword, include Carry
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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Examples
22
Binary Multiplication
• We already know that SHL performs unsigned
multiplication efficiently when the multiplier is a power
of 2.
• You can factor any binary number into powers of 2.
• For example, to multiply EAX * 36, factor 36 into 32 + 4
and use the distributive property of multiplication to
carry out the operation:
EAX * 36
= EAX * (32 + 4)
= (EAX * 32)+(EAX * 4)
mov
mov
shl
shl
add
eax,123
ebx,eax
eax,5
ebx,2
eax,ebx
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
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; mult by 25
; mult by 22
Examples
23
Your turn . . .
Multiply AX by 26, using shifting and addition instructions.
Hint: 26 = 16 + 8 + 2.
mov ax,2
mov dx,ax
shl dx,4
push dx
mov dx,ax
shl dx,3
shl ax,1
add ax,dx
pop dx
add ax,dx
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
; test value
; AX * 16
; save for later
;
;
;
;
;
AX * 8
AX * 2
AX * 10
recall AX * 16
AX * 26
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Examples
24
Isolating a Bit String
• The MS-DOS file date field packs the year, month,
and day into 16 bits:
DH
DL
0 0 1 0 0 1 1 0
Field:
Bit numbers:
Year
9-15
0 1 1 0 1 0 1 0
Month
5-8
Day
0-4
Isolate the Month field:
mov
shr
and
mov
ax,dx
ax,5
al,00001111b
month,al
Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.
;
;
;
;
make a copy of DX
shift right 5 bits
clear bits 4-7
save in month variable
Web site
Examples
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