SAL – Part 1

Simple Abstract Language

What do we need in a language?

The language must provide a way to specify the type of the variable

It must be able to specify arithmetic operations, e.g., add, subtract,etc.

It must provide control structures that allow looping and conditional execution.

It must provide a way to communicate with a user of the program.

Why write assembly language programs?

There are features of the computer that can be accessed with assembly language that are not well-captured in a high level language, e.g. critical parts of an operating system

To satisfy critical constraints of a program, e.g., it must fit in a very small amount of memory

Unavailability of a good compiler Compiler writers need to

understand how to write programs in assembly language before they can even write compilers

Variable Declaration SAL understands three simple types:

integer .word character .byte real .float

SAL understands one complex type: string .asciiz

The declaration is accomplished by giving a variable name and a type.

{ variablename: } .word {value}

Example 2.1

C++int temp;

int i = 5;

SALtemp: .word

i: .word 5

Example 2.2

sentinel: .byte ‘z’

linefeed: .byte ‘\n’

amount: .float 136.42

amount: .float 1.2642E2

amount: .float10.13642e+3

Example 2.3

string: .asciiz “Hello World!”str2: .asciiz “Goodbye Cruel

World…”

Instructions and Variables in SAL

The memory is divided into two distinct areas: area for variables a.k.a. data space area for instructions a.k.a. code or text

space In SAL, declarations can occur

anywhere, but they must be separated from code by the use of directives or pseudo-instructions.

The code space is distinguished in SAL by preceding it with

.text

The data space is distinguished in SAL by preceding it with

.data

Arithmetic Operations

A SAL instruction consists of an operation specification, known as opcode, and two or three operand specification.

move x,y x = y;

add x,y,z x = y + z;

sub x,y,z x = y - z;

mul x,y,z x = y * z;

div x,y,z x = y / z;

rem x,y,z x = y % z;

Example 2.4

areatri = (width*height)/2;

areatri: .float

width : .float

height: .float

tmp: .float

mul tmp,width,height

div areatri,tmp,2.0

Example 2.5

.data #data space begins

areatri:.float #declare areatri as float

width: .float 10.0 #declare and initialize width

height: .float 5.0 #declare and initialize height

tmp: .float #declare tmp

.text #code space begins

__start: #begin execution -required label

mul tmp,width,height #tmp := width * height

div areatri,tmp,2.0 #aretri := tmp/2

put areatri #display the value of areatri

put '\n’ #put a newline character

done #a macro to indicate end of program

Example 2.6

Write a SAL program to find the cube of a number, say 5. Here’s the start:

.data

result: .float

base: .float 5.0

.text

__start:

????

Control Structures

C++ provides two categories of control structures: conditionals

for example, if-then statement iteratives

for example, while statement

In SAL, these control structures can be done using a construct called branch

SAL’s branch instructions

Listed below are some of SAL’s branch instructions. See Table 2.2 of your textbook for a complete list.

b label goto label

beq x,y,label if x==y then goto label

ble x,y,label if x<=y then goto label

bltz x,label if x<0 then goto label

bnez x,label if x!=0 then goto label

bgtz x,label if x>0 then goto label

Example 2.7

C++: if ( A > 0 )

B = C / A;

else

B = A + 10;

SAL: blez A, else

div B,C,A

b continue

else: add B,A,10

continue:

Example 2.8

C++: if ( A > 0 )

B = C * A;

else

B = A - 10;

SAL: bgtz ___, ifpart

___ ___,___,___

b continue

ifpart: ___ ___,___,___

continue:

Unconditional branch

An unconditional branch can also be constructed from a special case of a conditional branch

b next #unconditional branch to next

beqz 0, next #if 0=0 then goto next

Compound conditionalC++: if ((A==B) || (C<D)) {

A = A + 1;B = B - 1;}

SAL:beq A,B,do_ifblt C,D,do_ifb endif

do_if: add A,A,1sub B,B,1

endif:

Example 2.9

C++: if (((A==B) && (C==D)) || (E<0)) {

A = A + 1;

C = E;

}

SAL:bne A,B,check_E

beq C,D,do_if

check_E: bgez E,end_if

do_if: add A,A,1

move C,E

end_if:

Loop induction

Unlike HLLs, code in SAL must contain an instruction to increment the loop variable.

C++:int base = 5, exp=4, result=1;for (int i =0; i < exp; i++)

result*=base;

result: .word 1

counter: .word 1

exp: .word 4

base: .word 5

for: bgt counter,exp,endfor

mul result,result,base

add counter,counter,1 b for

endfor:

How to run your SAL programs

Create a text file containing your code.

On the miller prompt, run the simulator by typing:

spimsal To load your program, type

load “filename”

To run your program, typerun

To exit spimsal, typeexit

Using script to record your sessions

On miller, type: script {filename} To display your code, type: cat myfile

Run spimsal, then load and run your program

Type exit, to exit script You may be asked to submit the file filename

A quick review in Unix

text editors - pico, emacs, vi cp - copy file, mv - move files redirection: >, >>, < ftp - file transfer protocol telnet sessions ls - list files pwd - check your current directory