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EEC70, Fall 2007
Instructor: Chuah
Instructor: Chen‐Nee Chuah 3125 Kemper Hall Lecture: M/W/F 11am‐12noon Office Hours: (M) 12:30‐1:30pm, (W) 10 ‐11am ***** Teaching Assistants: Victor Yip, Kurtis Kredo
Chapter 2: Simple Abstract Language
Variable Declaration Arithmetic Operations Control Structures Communication with users, I/O Procedures Sample Program
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Review Questions
What are the basic components of a computer? What is a program counter? What are the 6 steps involved in the instruction execution sequence? How do you implement branches? How many operands can an assembly language instruction (or statement) have? What is a typical syntax of MIPS instructions? What is instruction set architecture?
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Reminder: Translation from HLL to
Machine Code
SAL:SAL: Simple AbstractSimple Abstract LanguageLanguage (Chapter 2)(Chapter 2)
MAL:MAL: MIPS AssemblyMIPS Assembly LanguageLanguage (Chapter 8)(Chapter 8)
TAL:TAL: True AssemblyTrue Assembly LanguageLanguage (Chapter 10)(Chapter 10)
HLL Compiler^
Assembly Language Assembler^
Machine Code
Focus on this class
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Motivation for SAL
SAL is an intermediate‐level language, makes the transition from high‐level language to low‐level (machine) language easier to understand ‐ Fairly easy to understand (like HLL) and straightforward to translate into MAL (or TAL) ‐ Specific details of the machine language can be postponed
Progression of learning (with increasing detail):
HLLHLL^ SALSAL^ MALMAL^ TALTAL Machine CodeMachine Code
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SAL’s Top‐Level Syntax
One instruction or data declaration per line
Instructions have at most two operands and produce one result ‐ Some instructions have a single operand
Comments are anything on a line that follows “#”. Multiple‐line comments require “#” on each line
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SAL Data Declarations
Indicate how much memory space a data item needs
Allow a name (label) to be assigned to that memory space
Can declare 3 different data types: integer, real, and character
Example:
C/Java: C/Java: intint area;area; SAL:SAL: area:area: .word.word
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Declarations (Continued)
SAL declaration syntax: variable_name: type initial_value
types are: .byte, .word, and .float
Examples: count: .word 0 yes: .byte ‘y’ pi: .float 3.
Initial values are always defined ‐ default value is 0 for .word and .float ‐ “null” character for .byte
Question: So, how many bits really are there in a byte? word? float?
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Declarations (Continued)
Format for initial .float values is: {+,‐} {digits}. {digits} {e,E} {+,‐} {digits}
Examples that are the same value:
1.23456E +12.3456e 0.123456e+ 123456E‐ 3
One declaration per line
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MACRO: indicate program has completed, Allowing next program to run
Example
.data avg .word i1: .word 20 i2: .word 13 i3: .word 82 .text _start: add avg, i1, i add avg, avg, i div avg, avg, 3 done
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SAL SAL C/JavaC/Java
move x,ymove x,y x= y;x= y; add x,y,zadd x,y,z x= y+z;x= y+z; sub x,y,zsub x,y,z x= y-x= y-z;z; mul x,y,zmul x,y,z x= y z;x= y z; div x,y,zdiv x,y,z x= y / z;x= y / z; rem x,y,zrem x,y,z x= y % z;x= y % z;
Arithmetic Operations
SAL’s arithmetic operations are similar to HLL arithmetic operations, except the number of operands is limited to 2 or 1 ‐ Syntax is different
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Arithmetic Operations (cont.)
Operands can be a variable (represented by a label), or a constant. Examples: move count, 0 mul product,mult1,mult add sum,this,that SAL also includes Boolean operations (and, or, xor, etc.) which we’ll cover later
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Conditional Execution
Sometimes an instruction should be executed, sometimes not In C/Java: if (condition) statement else statement SAL has conditional branches (“conditional gotos”) General forms: b cond x,y label # if cond (x,y) is true, then branch (goto) label b cond x,0 label # if cond (x,0) is true, then branch (goto) label b label # branch unconditionally to label
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Compiler Optimization
Compilers use many interesting methods for producing assembly code that is fast and compact. One optimization, reversing the comparison, can be effective for the if‐then‐else example: bgez count,endif add count,count, endif: # next instruction goes here
One instruction (unconditional branch) is eliminated
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IF‐Else
Example C/Java IF: if (A > 0) B = C + D else B = E + F
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IF‐Else (cont)
One SAL implementation: blez A, else add B, C, D b endif else: add B, E, F endif: Second SAL implementation: bgtz A, if add B, E, F b endif if: add B,C,D endif: Which is best, or is another best?
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IF‐Else (cont)
One SAL implementation: blez A, else add B, C, D b endif else: add B, E, F endif: Second SAL implementation: bgtz A, if add B, E, F b endif if: add B,C,D endif: Which is best, or is another best?
A>0 A>
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IF‐Else (cont)
One SAL implementation: blez A, else add B, C, D b endif else: add B, E, F endif: Second SAL implementation: bgtz A, if add B, E, F b endif if: add B,C,D endif: Which is best, or is another best?
A>0 A>
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IF‐Else (cont)
One SAL implementation: blez A, else add B, C, D b endif else: add B, E, F endif: Second SAL implementation: bgtz A, if add B, E, F b endif if: add B,C,D endif: Which is best, or is another best?
A>0 A>
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IF‐Else (cont)
C/Java IF: if (A > 0) B = C + D else B = E + F Is the same as: B = E + F if (A > 0) B = C + D SAL: add B, E, F blez A, endif add B,C,D endif: Reduce one line of code!
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Breakpoint
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Loop Construction Example: While
C/Java while (count > 0) { a = a % count; count := count -1; }
SAL: while: blez count, endwhile rem a,a,count add count,count,- b while endwhile: # next instruction here
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Loop Construction Example: For
C/Java: for (i=0; i<10; i++) a = a+i;
SAL:
move x,10move x, move i,0move i, for:for:^ bge i,x, endforbge i,x, endfor add a,a,iadd a,a,i add i,i,1add i,i, b forb for endfor:endfor: # next instruction goes here# next instruction goes here
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SAL’s Input Output Operations
It is necessary for a program to communicate with the outside world ‐ the user, via keyboard and display ‐ attached peripherals, e.g., printer, scanner, microphone, speaker, etc. ‐ other networked computers SAL has a very simple set of input/output commands for keyboard and character display I/O ‐ put x # value of x is written to display ‐ get x # x is assigned the value input on keyboard ‐ puts string # writes a character string to display
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SAL’s I/O Abstraction
my program
Processor
Memory strings reals integers
chars reals integers
- Abstraction suggests keyboard and display use base 10 integer and real numbers, and alphanumeric characters
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String Declarations (cont.)
String label indicates memory location of first character SAL puts command calls an I/O program, passing it the label address By agreement, I/O program knows where string ends when it finds a null character, ‘\0’. Thus: prompt:.ascii “Type in your name\0” For convenience .asciiz directive automatically adds null character at end of string. Thus: prompt:.asciiz “Type in your name”
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Oddities of get
get intvar places the first integer variable on the line into intvar, and then discards the rest of the line. Example:
Input: 23 ax 467 ‐ 14 735 1234abc!
SAL code: get int1 # int1 I 23 get int2 # int2 I ‐ 14 get int3 # int3 I 1234
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Example I/O Program: adds 2 Integers
.data prompt: .asciiz "Enter an integer: " linefeed: .byte '\n' message: .asciiz "The sum is " int1: .word int2: .word sum: .word .text __start: puts prompt # get an integer from user get int put linefeed puts prompt # get a integer from user get int put linefeed add sum, int1, int2 # calculate the sum puts message # print out the sum put sum put linefeed done
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Example
What happens when you use .ascii instead of .asciiz and forget to put “\0” at the end of the string declaration
