Download assembler

Computer Science 101
Assembly Language
Problems with Machine Language

Uses binary - No English-like words to make it
more readable

Only numeric memory addresses - Can not name
an instruction or data location

Difficult to change - If we leave out one
instruction, all addresses from that point on will
be incorrect.

Difficult to create data - Must use internal binary
representation
Assembly Language to the
Rescue!

Symbolic operation codes: Load, Store,

Symbolic names (user defined) for memory
addresses
Jump, Compare, etc.
Load Pay

Pseudo-operations provide services such as
data generation.
Assembler

An assembler is a program that takes a
source code program written in
assembly language and converts it to a
machine language object file.

Essentially a line by line translation.
Assembly lang
Source file
Assembler
Machine lang
Object file
Labels: Giving names to locations.

We can attach a symbolic name to any
instruction or data location by beginning the line
with a label with that name.

A label consists of the name followed by a colon.

You use labels in all of the places that addresses
would occur in machine language; i.e. you no
longer use numeric addresses.
– Address fields in instructions
 Operands of computations
 Targets of branch instructions
Pseudo-operations

Pseudo-operations are not statements that
are converted to machine code. They are
used to request services of the assembler.
Performed only when program assembled.

Hypothetical Machine Pseudo-Ops:
All of the pseudo-ops begin with a period.
.begin
-- required to mark beginning
.end
-- required to mark end
.data 30
-- puts a value at the memory
location; value in decimal
assembler converts to binary
Example: Add X to Y and Store in Z
Z=X+Y
.begin
load X
add Y
store Z
halt
X: .data 33
Y: .data 40
Z: .data 0
.end
Developing Assembly Programs

First write your algorithm in pseudocode.

Then convert to assembly.

Convert computations using labeled memory
locations and the register for variables document these.

We’ll look at techniques for converting
common control flow constructs to assembly
language.
Converting Set (Assignment)
Statements

Set <variable> to <expression>

Semantics:
Compute value of the expression
Give this value to the variable

Assembly language:
Accumulate value of expression in R
Store the value in the variable’s location
Example
Set X to 2Y + Z – 5

load Y
add Y
add Z
subtract FIVE
store X
…
X:
.data 0
Y:
.data 20
Z:
.data 30
FIVE: .data 5
…

Converting If-statements
If XY then
Statements A
Statements B
Cond
T
load X
compare Y
jumpgt BSpot
…
Translation of A
...
A Code
F
B Code
BSpot: …
Translation of B
If-statement example
.begin
User inputs number.
If number is greater
than 5 we output
the number.
 Pseudocode:
Get N
If N>5 then
Print N
Stop

DONE:
N:
FIVE:
.end
in N
load N
compare FIVE
jumpgt DONE
jumpeq DONE
out N
halt
.data 0
.data 5
Converting If-else-statements
If XY then
Statements A
else
Statements B
Statements C
load X
compare Y
jumpgt Else
…
Translation of A
F
BCode
Cond T
A Code
...
jump CSpot
Else: …
Translation of B
…
…
C Code
CSpot: Translation of C
Converting While-statements
While XY Do
Statements A
Statements B
Cond
T
WHILE:
load X
compare Y
jumpgt BSpot
…
Translation of A
...
jump WHILE
F
A Code
BSpot: …
Translation of B
B Code
…
While-statement example


Program outputs sum of 10
numbers entered by the
user.
Pseudocode:
Set Ct to 10
Set Sum to 0
While Ct>0
Get Num
Set Sum to Sum+Num
Set Ct to Ct-1
Print Sum
Stop
.begin
WHILE:
PRNT:
load TEN
store CT
clear SUM
load CT
compare ZERO
jumpeq PRNT
in NUM
load NUM
add SUM
store SUM
decrement CT
jump WHILE
out SUM
halt
While-statement example
(cont.)
.begin
WHILE:
PRNT:
load TEN
store CT
clear Sum
load CT
compare ZERO
jumpeq PRNT
in NUM
load NUM
add SUM
store SUM
decrement CT
jump WHILE
out SUM
halt
TEN:
CT:
SUM:
NUM:
ZERO:
.end
.data 10
.data 0
.data 0
.data 0
.data 0