Download ADD and SUB Instructions

CS2422 Assembly Language and System Programming
Week 9
Data Transfers, Addressing, and
Arithmetic
0
What's Next

Addition and Subtraction
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INC and DEC Instructions
ADD and SUB Instructions
NEG Instruction
Implementing Arithmetic Expressions
Flags Affected by Arithmetic
‒ Zero, Sign, Carry, Overflow
1
INC and DEC Instructions

Add 1/subtract 1 from destination operand
operand may be register or memory
.data
myWord WORD 1000h
myDword DWORD 10000000h
.code
inc myWord
; 1001h
dec myWord
; 1000h
inc myDword
; 10000001h
mov ax,myDword
inc ax
; AX = 10000001h

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ADD and SUB Instructions
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
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ADD destination, source
 Logic: destination  destination + source
SUB destination, source
 Logic: destination  destination – source
Same operand rules as for MOV instruction
.data
var1 DWORD 10000h
var2 DWORD 20000h
.code
mov eax,var1
add eax,var2
add ax,0FFFFh
add eax,1
sub ax,1
;
;
;
;
;
;
---EAX--00010000h
00030000h
0003FFFFh
00040000h
0004FFFFh
3
NEG (negate) Instruction

Reverses the sign of an operand. Operand can
be a register or memory operand
.data
valB BYTE -1
valW WORD +32767
.code
mov al,valB
neg al
neg valW
; AL = -1
; AL = +1
; valW = -32767
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Arith. Expression in Assembly

HLL mathematical expressions are translated
into assembly language by compiler, e.g.
Rval = -Xval + (Yval – Zval)
Rval DWORD ?
Xval DWORD 26
Yval DWORD 30
Zval DWORD 40
.code
mov eax,Xval
neg eax
; EAX = -26
mov ebx,Yval
sub ebx,Zval
; EBX = -10
add eax,ebx
mov Rval,eax
; -36
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Flags Affected by Arithmetic

ALU has a number of status flags that reflect the
outcome of arithmetic (and bitwise) operations
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Essential flags:
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based on the contents of the destination operand
Zero: set when destination equals zero
Sign: set when destination is negative
Carry: set when unsigned value is out of range
Overflow: set when signed value is out of range
The MOV instruction never affects the flags
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Zero Flag (ZF)

Zero Flag is set when the result of an operation
produces zero in the destination operand
mov
sub
mov
inc
inc
cx,1
cx,1
ax,0FFFFh
ax
ax
; CX = 0, ZF = 1
; AX = 0, ZF = 1
; AX = 1, ZF = 0
Remember...
• A flag is set when it equals 1.
• A flag is clear when it equals 0.
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Sign Flag (SF)

Sign Flag is set when the destination operand is
negative and clear when destination is positive
mov cx,0
sub cx,1
add cx,2

; CX = -1, SF = 1
; CX = 1, SF = 0
Sign flag is a copy of the destination's highest bit:
mov al,0
sub al,1
add al,2
; AL = 11111111b, SF = 1
; AL = 00000001b, SF = 0
8
Carry Flag (CF)

The Carry Flag is set when the result of an
operation generates an unsigned value that is
out of range (too big or too small for the
destination operand)  carry or borrow
mov al,0FFh
add al,1
; CF = 1, AL = 00
; Try to go below zero:
mov al,0
sub al,1
; CF = 1, AL = FF
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Overflow Flag (OF)

The Overflow Flag is set when the signed result
of an operation is invalid or out of range
; Example 1
mov al,+127
add al,1
; Example 2
mov al,7Fh
add al,1
; OF = 1,
AL = ??
; OF = 1,
AL = 80h
The two examples are identical at binary level because
7Fh equals +127. To determine the value of destination
operand, it is often easier to calculate in hexadecimal.
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Exercise

Run Example AddSub3

Output
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CS2422 Assembly Language and System Programming
Week 10
Data Transfers, Addressing, and
Arithmetic
12
What's Next

Data Transfer Instructions
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Operand Types
MOV Instruction
Direct Memory Operands
Direct-Offset Operands
Zero and Sign Extension
XCHG Instruction
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Operand Types

Three basic types of operands:

Immediate – a constant integer (8, 16, or 32 bits)
‒ value is encoded within the instruction

Register – the name of a register
‒ register name is converted to a number and
encoded within the instruction

Memory – reference to a location in memory
‒ memory address is encoded within the instruction,
or a register holds the address of a memory
location
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Instruction Operand Notation
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Data Transfer Instructions

MOV is for moving data between:
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Memory
Register
Immediate (constant)
Almost all combinations, except:

Memory to Memory!
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MOV Instruction
• Move from source to destination. Syntax:
MOV destination, source
• Source and destination are of same size
• No more than one memory operand permitted
• CS, EIP, and IP cannot be the destination
• No immediate to segment register moves
.data
count BYTE 100
wVal WORD 2
.code
mov bl,count
mov ax,wVal
mov count,al
mov al,wVal
mov ax,count
mov eax,count
; error
; error
; error
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Your turn . . .
Explain why each of the following MOV statements are invalid:
.data
bVal BYTE
100
bVal2 BYTE
?
wVal WORD
2
dVal DWORD 5
.code
mov ds,45
mov esi,wVal
mov eip,dVal
mov 25,bVal
mov bVal2,bVal
;immediate move to DS not permitted
;size mismatch
;EIP cannot be the destination
;immediate value cannot be destination
;memory-to-memory move not permitted
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Memory to Memory?

Must go through a register…
.data
Var1 WORD 100h
Var2 WORD ?
.code
MOV ax, var1
MOV var2, ax
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Zero or Sign Extension

What happens to ECX if –1 is moved to CX?
.data
signedVal SWORD -16
.code
mov ecx,0
mov cx,signedVal
 Are the higher 16 bits of ECX all 0?
 What number does ECX represent now?
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The solution: MOVZX and MOVSX
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MOVZX always fills higher bits with 0.
MOVSX fills higher bits by “sign extension”.
Just extend the left-most bit!
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Zero Extension

When copy a smaller value into a larger
destination, MOVZX instruction fills (extends) the
upper half of the destination with zeros
0
10001111
Source
(bl)
00000000
10001111
Destination
(ax)
mov bl,10001111b
movzx ax,bl
; zero-extension
The destination must be a register
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Sign Extension

MOVSX fills the upper half of the destination with
a copy of the source operand's sign bit
10001111
Source
10001111
Destination (ax)
Does it affect
the value?
11111111
(bl)
mov bl,10001111b
movsx ax,bl
; sign extension
The destination must be a register
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LAHF/SAHF and XCHG

LAHF to load flags (EFLAGS) into AH
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Loads Sign, Zero, Auxiliary Carry, Parity, Carry
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SAHF to store contents of AH to EFLAGS
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XCHG for exchanging data between:
Register, register
 Register, memory
 Memory, register
(again, no memory to memory)

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XCHG Instruction

XCHG exchanges the values of two operands.
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At least one operand must be a register.
No immediate operands are permitted
.data
var1 WORD 1000h
var2 WORD 2000h
.code
xchg ax,bx
xchg ah,al
xchg var1,bx
xchg eax,ebx
;
;
;
;
xchg var1,var2
;error: two memory operands
exchange
exchange
exchange
exchange
16-bit regs
8-bit regs
mem, reg
32-bit regs
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