1. Assembler Design Options
CSCI/CMPE 3334
David Egle

2. Design Options Have studied the two pass assembler One pass assemblers
not the only design possible
One pass assemblers
useful when it is necessary or desirable to avoid a second pass
Multi-pass assemblers
an extension to the two pass assembler that allows the assembler to handle forward references in symbolic expressions

3. One Pass Assembler Does everything in one pass
Problem: How do we handle forward references?
Could eliminate forward references
easy for data – just define the data areas before they are referenced
not easy in code – how do we handle selection or loop statements which have forward jumps?
Two types of one pass assemblers:
produce object code directly in memory
produce object file for later linking/loading/execution

4. Load and go assembler
Generate object code in memory for immediate execution
No loader is needed
More efficient
Avoids the overhead of writing the object program out and reading it and loading it back in
Avoids overhead of an additional pass over the source
Also, storage devices for the intermediate file might not be available, slow, or inconvenient

5. Operation of load and go
Assembler generates object code instructions as it scans source program
If an operand symbol has not yet been defined
operand address is set to 0 in instruction
symbol is entered into the symbol table (unless it is already present)
entry is flagged to indicate the symbol is undefined
address of instruction is added to list of forward references associated with this symbol
When symbol definition is encountered
forward reference list is scanned, and proper address is inserted in any instructions previously generated (in memory)

6. One pass assembler - Algorithm
read first input line
if OPCODE = ‘START’ then
{ save #{OPERAND} as starting address
initialize LOCCTR to starting address
read next input line
else set LOCCTR=0
while OPCODE != ‘END’ do
{ if this is not a comment line then
handle line }
save (LOCCTR – starting address) as program length
Put value of OPERAND on END directive in PC

7. Algorithm - 2 If there is a symbol in the LABEL field then
search SYMTAB for LABEL
if found then
if undefined
mark defined; go through reference list and fix up memory locations
else
set error flag (duplicate label)
else insert (LABEL, LOCCTR) into SYMTAB
verify mnemonic and get value
if operand is present
search SYMTAB for OPERAND
if defined
get the address
set address to 0 and add current value of LOCCTR to reference list
insert OPERAND in symbol table, marked undefined, and add LOCCTR to reference list
Build machine code
Handle directives as appropriate
Insert code in memory

8. One pass assembler that produces object programs
Use the same procedure
When the definition of a symbol is encountered
if instruction which made the forward reference is still in memory, then fix it
if not, the instruction has already been written out in a Text record, so generate a new Text record with the correct operand address
the loader will fix up the address field

9. Multi-pass assemblers
Required if we remove the restriction that all the symbols on the right hand side of an EQU or ORG directive must be defined
Note that removing this restriction is not really a convenience for the programmer as it leads to unreadable code!
But, if removed, the multiple passes are use to gradually evaluate the expressions
The number of passes will depend on the depth of forward referencing

10. Implementation examples
MASM assembler
SPARC assembler
AIX assembler

11. Review problems – Chapter 2
Section 2.1
2, 7
Section 2.2
1, 2, 3
Section 2.3
4, 7, 8, 11, 16, 17, 20, 22
Section 2.4
6, 8, 9