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Chapter 7Chapter 7
LC-3 Assembly Language
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Wright State University, College of Engineering
Dr. Doom, Computer Science & Engineering CEG 320/520
Comp. Org. & Assembly
CS RealityCS Reality
You’ve got to know assembly
Chances are, you’ll never write program in assembly
– Compilers are much better & more patient than you are
Understanding assembly key to machine-level execution model
– Behavior of programs in presence of bugs
High-level language model breaks down
– Tuning program performance
Understanding sources of program inefficiency
– Implementing system software
Compiler has machine code as target
Operating systems must manage process state
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Wright State University, College of Engineering
Dr. Doom, Computer Science & Engineering CEG 320/520
Comp. Org. & Assembly
It’s hard to write code in 1’s & 0’s!It’s hard to write code in 1’s & 0’s!
Assembly language makes it possible to write Machine Language code
– each line of assembly language is translated into a single ML instruction
A program called the Assembler does the translation and provides useful
tools:
– use of labels - symbolic names for address locations
– automatic conversion of binary / hex / decimal
– pseudo-ops
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Wright State University, College of Engineering
Dr. Doom, Computer Science & Engineering CEG 320/520
Comp. Org. & Assembly
Assembly Language InstructionsAssembly Language Instructions
Formats
– LABEL OPCODE OPERANDS ; COMMENTS
– LABEL PSEUDO-OPS ; COMMENTS
Opcode
– Symbolic name for the 4-bit ML opcode
Label
– Symbolic name for a memory location. It is used to:
indicate the target of a branch instruction, e.g. AGAIN in location 0B
indicate the location of a stored value or array, e.g. NUMBER and SIX
Comments
– intended for humans only: explanation of code, visual display
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Wright State University, College of Engineering
Dr. Doom, Computer Science & Engineering CEG 320/520
Comp. Org. & Assembly
PseudoPseudo--Ops … Ops …
– … are directives to the assembler
they are not translated into ML instructions
– LC-3 Pseudo-Ops:
.ORIG address Tells assembler where to locate the program in
memory (starting address).
.FILL value Store value in the next location in memory.
.BLKW n Set aside a block of n words in memory.
.STRINGZ string Store the string, one character per word, in
memory. Add a word of x0000 after the string.
.END Marks the end of the source program (not to be
confused with the instruction HALT!)
.EXTERNAL The label so indicated is allocated in another module.
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Wright State University, College of Engineering
Dr. Doom, Computer Science & Engineering CEG 320/520
Comp. Org. & Assembly
A sample programA sample program
01 ;
02 ; Program to multiply an integer by the number 6
03 ;
04 .ORIG x3050
05 LD R1, SIX
06 LD R2, NUMBER
07 AND R3, R3, #0 ; clear R3 to hold the product
08 ;
09 ; The inner loop
0A ;
0B AGAIN ADD R3, R3, R2
0C ADD R1, R1, #-1 ; keep track of iterations
0D BRp AGAIN
0E ;
0F HALT
10 ;
11 NUMBER .BLKW 1
12 SIX .FILL x0006
13 ;
14 .END
