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name
operation
START: MOV
operand(s)
CX, 5
;comment
;
;
যা ইচ্ছা লিখ! ‘ ’ লিও!
name
operation
operand(s)
• Okay  letters, digits, ?, ., @, _, $, %
• Illegal names –
-
If any blanks – aaa aaa
If begin with a digit – 2asad
If ‘.’ not first char. – aq.12
If has illegal char – aa&bb
;comment
name
operation
operand(s)
;comment
• For an Instruction: Symbolic operation code
[opcode]:
Assembler translates these codes into Machine
lang opcode.
 MOV, ADD, SUB, …
• For an Directive: pseudo-operation code –
pseudo-op, do not translate
 PROC is used to create a procedure
name
operation
operand(s)
;comment
• An instruction may have 0/1/2 operands
e.g.,
NOP
; no operands – does nothing
INC
AX
; 1 op. – adds 1 to contents of AX
ADD
destination, source
4.2 Program data
• Binary – 1011b, 1100B
• Decimal – 10111, 10111d, -3419D
1,23 – x
• Hex – start with a decimal digit, ends with ‘H’
– 3BA3H, 0FFFFH
• Characters – within ‘aa’ or “aa”
Pseudo-ops
•
•
•
•
•
DB – define data BYTE
DW – word
DD – doubleword – two consecutive words
DQ – quadword – 4 …
DT – tenbytes – 10…
4.3 Variables
name
operation
name
e.g.,
ALPHA
AA
DB
DB
initial value
4
DB
operand(s)
;comment
; -128 ~ 127; or 0-255
; variable ALPHA=4
?
; uninitialized byte
4.3.3 Array
EktaArray
Symbol
EktaArray
EktaArray+1
EktaArray+2
DB
10H, 20H, 22H
Address
200h
Contents
10H
201h
202h
20H
22H
For DW – 2 bytes
Symbol
EktaArray
EktaArray+2
EktaArray+4
Address
200h
Contents
10H
202h
204h
20H
22H
WORD1
DW
1234H
; low Byte of WORD1 contains 34H
; high Byte of WORD1 contains 12H
4.5 basic instructions
• MOV – move; contents of destination is
replaced by the contents of source
• XCHG – exchange; swapping
Q: exa??
• ADD
W1, AX
;W1 = W1 + AX
;W1 – changes with added/sum value
;AX – unchanged
• SUB
AX, DX
;Subtract DX from AX
;New value in AX – AX changes
; DX unchanged
Direct ADD/SUB between memory locations  WRONG!
• ADD
W1, W2
; wrong as direct memory location
Source
Gen Reg.
Memory Loc.
Constant
Destination
General Reg.
Yes
Yes
Yes
Memory loc.
Yes
NO
Yes
INC
Increment
• INC
INC
destination
W1
;e.g., W1=0003
Before
After INC,
W1=0003
W1=????
0004
DEC
Decrement
• DEC
DEC
destination
W1
;e.g., W1=FFFF
Before
After INC,
W1=FFFF
W1=????
FFFE
NEG
negate the contents of destination
• NEG
destination
; reg. / mem.
; replace the contents by it’s two’s complement
NEG
Before
After NEG,
W1
;e.g., W1=0002
W1=0002
W1=????
FFFE
• Note:
- Both types MUST be the same type
e.g.,
MOV
AH, ‘A’
; what is in AH?
; ‘A’ = 41H
41H
MOV
AX, ‘A’ ; what is in AX?
0041H
Do!
High- to Assembly
1. B = A
2. A = 5 – A
3. A = B – 2 x A
1. B = A
• MOV
• MOV
AX, A
B, AX
• A direct mem-to-mem move is illegal
2. A = 5 - A
• MOV
• SUB
• MOV
AX, 5
AX, A
A, AX
• NEG
• ADD
A
A, 5
3. A = B – 2xA
• MOV
• SUB
• SUB
AX, B ;AX = B
AX, A;AX=B-A
AX, A;AX=B-A – A
; AX = B – 2A
• MOV
A, AX; A = AX = B – 2A