L08 [Assembly]

Anytime you have a PC relative addressing mode (e.g. PCOffset), we use a label.

The Assembly Process

• Objective
  • Translate the Assembly Language program into ML (Machine Language)
  • Each AL instruction yields one ML instruction word
  • Interpret pseudo-ops correctly
• Problem
  • An instruction may reference a label
  • If the label hasn't been encountered yet, the assembler can't form the instruction word
• Solution
  • Two pass assembly

First Pass [Generate Symbol Table]

• Scan each line
• Keep track of current address (location counter)
  • Increment by 1 for each instruction
  • Adjust the location counter as required for any pseudo-ops (e.g. .fill or .stringz)
• For each label, enter it into the symbol table along with the current address
• Stop when .end is encountered

Second Pass [Generate the Machine Language Program]

• Scan each line again
• Translate each AL instruction into ML
  • Look up symbols in the symbol table
  • Ensure that labels are no more than +256 / -255 lines from PCOffset9 instructions
  • Calculate operand field for the instruction
  • Update the current address (location counter)
• Fill memory locations as directed by pseudo-ops
• Stop when .end is encountered

Aliases

TRAP pseudo-instruction

• The assembler will recognize aliases for certain predefined trap instructions:

Alias	Instruction
TRAP x25	HALT	Stop the CPU
TRAP x23	IN	Input character from keyboard
TRAP x22	PUTS	Out
TRAP x24	PUTSP
TRAP x20	GETC

RET alias

RET is just an alias for JMP R7

Need LD?

LD R1, Data


...

DATA. fill 5

Just becomes

LEA R1, DATA
LDR R1, R1, #0


...

DATA .fill 5

Need LDI?

LDI R1, ADDR


...

ADDR .fill xFE00

becomes

LEA R1, ADDR

LDR R1, R1, #0
LDR R1, R1, #0

...

ADDR .fill xFE00

Converting Between Assembly And Hex

.ORIG x3000
    LEA R0, ARRAY
    LDR R1, R0, #5
    ADD R1, R1, #4
    ST R1, ARRAY
    HALT
.END

.ORIG x3020
ARRAY
.fill 7
.fill 2
.fill 9
.fill 10
.fill 4
.fill 11

Try converting ST R1, ARRAY into its hexadecimal value!

Solution

• Let's start by looking at the op code for ST
  • The op code for ST is 0011
• The SR is R1, which is 001
• Now, we just have to calculate PCOffset9
  • ARRAY is at 0x3020
  • Current PC is 0x3004
  • ARRAY - current PC = 0x1C = 0b000011100
• Putting this together, we get 0011-001-000011100 or 0xE60F