Download Assembly Language Operation

8086 Programming
Addition Instruction
The table illustrates the addressing modes available to the ADD
instruction. These addressing modes include almost all addressing
modes.
The only types of addition not allowed are memory-to-memory
and segment register.
The segment registers can only be moved, pushed, or popped.
Assembly Language
Operation
ADD AL,BL
ADD CX,DI
ADD CL,44H
AL
CX
CL
AL + BL
CX + Dl
CL + 44H
ADD BX,245FH
BX
BX + 245FH
ADD [BX],AL
ADD CL,[BP]
[DS:BX]
[DS:BX] + AL
CL
CL + [SS:BP]
ADD BX,[SI + 2]
BX
ADD CL,TEMP
ADD BX, TEMP[DI]
CL
BX
ADD [BX + DI],DL
ADD BYTE PTR [DI],3
BX + [DS:(SI + 3:SI + 2)]
CL + [DS:TEMP]
BX +[DS:(TEMP+DI + 1:
TEMP+DI)]
[BX + DI]
[BX + DI] + DL
[DI]
[DI] + 3
1
Increment Instruction
Increment instruction (INC) adds 1 to a register or a memory
location.
The INC can add 1 to any register or memory location, except a
segment register.
With indirect memory increments, the size of the data must be
described by using the BYTE PTR or WORD PTR directives.
The reason is that the assembler program cannot determine if, for
example, the INC [DI] instruction is a byte-, or word -sized
increment.
The INC BYTE PTR [DI] instruction clearly indicates byte-sized
memory data; the INC WORD PTR [DI] instruction indicates a
word-sized memory data
Assembly Language
INC BL
INC SP
INC BYTE PTR [BX]
INC WORD PTR [SI]
INC DATA1
Operation
BL
BL + 1
SP
SP + 1
[DS:BX]
[DS:BX] + 1
[DS:SI+1:SI]
[DS:SI+1:SI] + 1
[DS:DATA1]
[DS:DATA1] + 1
Increment instructions affect the flag bits except the carry flag bit.
Carry doesn't change because we often use increments in
programs that depend upon the contents of the carry flag.
Note that increment is used to point to the next memory element
in a byte-sized array of data only.
If word-sized data are addressed. it is better to use an ADD DI,2
instruction to modify the DI pointer in place of two INC DI
instructions.
In some cases, the carry flag must be preserved which may mean
that two INC instructions might appear in a program to modify a
pointer.
2
Addition-with-Carry
An addition-with-carry instruction (ADC) adds the bit in the carry
flag (C) to the operand data. This instruction mainly appears in
software that adds numbers that are wider than 16 bits in the
8086.
Assembly Language
ADC AL,AH
ADC CX,BX
ADC DH,[BX]
ADC BX,[BP + 2]
Operation
AL
CX
DH
BX
+ carry
AL + AH + carry
CX + BX + carry
DH + [DS:BX] + carry
BX + [SS:(BP + 3:BP + 2)
3
Subtraction
Many forms of subtraction (SUB) appear in the instruction set.
The only types of subtraction not allowed are memory-to-memory
and segment register subtractions. Like other arithmetic
instructions, the subtract instruction affects the flag bits.
Assembly Language
Operation
SUB
SUB
SUB
SUB
SUB
SUB
SUB
dest
dest,source
AX,SP
CH,6FH
AX,OCCCCH
[DI],CH
CH,[BP]
AH, TEMP
dest-source
Decrement Instruction (DEC)
DEC instructionsubtracts a 1 from a register or the contents of a
memory location.
The decrement indirect memory data instructions require BYTE
PTR, or WORD PTR.
DEC dest
DEC CX
DEC BYTE PTR [DI]
DEC WORD PTR[BP]
dest
dest - 1
Subtract-with-Borrow
A subtraction-with-borrow (SBB) instruction functions as a
regular subtraction, except that the carry flag (C), which holds the
borrow, also subtracts from the difference.
The most common use for this instruction is for subtractions that
are wider than 16 bits in the 8086.
Wide subtractions require that borrows propagate through the
subtraction, just as wide additions propagate the carry.
Like the SUB instruction, SBB affects the flags.
Notice that the immediate subtract from memory instruction in
4
this table requires a BYTE PTR , or WORD PTR directive.
Assembly Language Operation
SBB dest,source
dest
SBB CL,2
CL
SBB BYTE PTR[DI],3
dest – source - carry
CL – 2 - carry
SBB [DI],AL
SBB DI,[BP + 2]
5
Comparison
The comparison instruction (CMP) is a subtraction that changes
only the flag bits; the destination operand never changes.
A comparison is useful for checking the entire contents of a
register or a memory location against another value.
A CMP is normally followed by a conditional jump instruction,
which tests the condition of the flag bits.
The only disallowed forms of compare are memory-to-memory
and segment register compares.
Assembly Language
dest – sourse; update flags
CMP dest,source
CMP CL,BL
CMP AX,SP
CMP AX,2000H
CMP [DI],CH
CMP CL,[BP]
EXAMPLE
CMP
JAE
Operation
AL,10H
SUBER
;compare with 10H
;if 10H or above
6