Chapter 7: Assembly Language

The LC-3 Assembly Language

Base Format

LABEL  OPCODE   OPERANDS  ; COMMENTS

Opcodes and operands

ADD

Encoding:

0001 DR SR1 0 00 SR2
0001 DR SR1 1 imm5

Assembler format:

ADD DR, SR1, SR2
ADD DR, SR1, imm5

AND

Encoding:

0101 DR SR1 0 00 SR2
0101 DR SR1 1 imm5

Assembler format:

AND DR, SR1, SR2
AND DR, SR1, imm5

NOT

Encoding: 1001 DR SR 1 11111

Assembler format: NOT DR, SR

LEA

Encoding: 1110 DR PCoffset9

Assembler format: LEA DR, LABEL

LD

Encoding: 0010 DR PCoffset9

Assembler format: LD DR, LABEL

LDI

Encoding: 1010 DR PCoffset9

Assembler format: LDI DR, LABEL

LDR

Encoding: 0110 BaseR PCoffset9

Assembler format: LDR DR, BaseR, offset6

ST

Encoding: 0011 SR PCoffset9

Assembler format: ST SR, LABEL

STI

Encoding: 1011 SR PCoffset9

Assembler format: STI SR, LABEL

STR

Encoding: 0111 BaseR PCoffset9

Assembler format: STR SR, BaseR, offset6

JMP

Encoding: 1100 000 BaseR 000000

Assembler format: JMP BaseR

Note: RET is JMP R7, when BaseR is 111.

BR

Encoding: 0000 n z p PCoffset9

Assembler format: BR(n)(z)(p) LABEL

JSR / JSRR

Encoding:

0100 1 PCoffset11
0100 0 00 BaseR 000000

Assembler format:

JSR LABEL
JSRR BaseR

TRAP

Encoding: 1111 0000 trapvect8

Assembler format: TRAP trapvect8

RTI

Encoding: 1000 000000000000

Assembler format: RTI

Pseudo-ops (Assembler Directives)

.ORIG

.ORIG tells the assembler where to put the first instruction of the program in memory.

.FILL e.g. .FILL xABCD

.FILL tells the assembler to set aside the next location and fill it with the value of its operand.

.BLKW e.g. .BLKW #15

.BLKW tells the assembler to set aside a number of sequential locations, specified by its operand.

.STRINGZ e.g. .STRINGZ "Hello!"

.STRINGZ tells the assembler to initialize the next n+1 location with its operand, which is a string of length n, and the final location is initialized with x0000.

.END

.END tells the assembler that the assembly program ends here. Contents after it will be neglected.

The Assembly Process: A Two-Pass Process

1. Creating the Symbol Table

During the first pass, the assembler establishes correspondences of labels with their 16-bit memory addresses. It creates a symbol table for reference.

The current location is tracked by a location counter (LC). LC is incremented in every line corresponding to an actual instruction.

2. Generating the Machine Language Program

During the second pass, the assembler goes through the assembly program again with the help of the symbol table, and translates each line into machine instructions. It subtracts LC+1 (the incremented PC) from the corresponding address of the label to get the offset. If the offset is too big to fit in the instruction, an assembly error occurs.