1. Optional Assembler Features
CSCI/CMPE 3334
David Egle

2. Machine Independent Features
Features that are not related to machine architecture
Presence or absence of such features is related to issues such as programmer convenience and software environment
We consider
literals
symbol defining statements
expressions
program blocks
control sections and program linking

3. Literals
Constant operand written “literally” as part of the instruction
SIC/XE notation
prefix = is added to the literal value specification
use the same notation as the BYTE or WORD directive
example:
ENDFIL LDA =C’EOF’
WLOOP TD =X’05’
LDA =5

4. DECSystem 10 example
This uses the [ ] to enclose the literal – in this case, a subprogram GETCHR: SOSG IBUF+2 ; any chars left PUSHJ P, [IN N, ; read in more POPJ P, ; return if no errors STATZ N, ; check error JRST [MOVEI E, ERRMSG ; get the error message JRST ERRPRT] ; print the message JRST ENDFIL] ; EOF handler ILDB AC, IBUF+1 ; get char POPJ P, ; return

5. Immediate vs Literal Immediate addressing Literal
the operand value is assembled as part of the instruction
Literal
the assembler generates the specified value as a constant at some other memory address
the address of the generated constant is used as the target address for the machine instruction

6. Literal pools Assembler collects the literals in a buffer
At the end of the program, the assembler dumps all the constants into a ‘literal pool’, and assigns addresses
An assembler directive, such at LTORG, may be used to place the literal pool at some other location
that pool contains all literal constants since the previous LTORG
usually used to allow PC relative addressing instead of extended format instructions

7. Handling Literals Need a table – literal table (LITTAB)
contains literal name, operand value and length, and address in the literal buffer
may be organized as a hash table
when literal buffer is dumped (into the literal pool), the address is updated to the appropriate location
Created during pass 1
Dumped at end of pass (or earlier)

8. Duplicate Literals Could be recognized by
comparing the strings defining them
comparing the generated data
slight savings vs. added complexity
Only store one copy of the specified value
Identical names / different values problem
some literal values depend on their location
the literal refers to an item whose value changes
e.g., the symbol * refers to the current LOCCTR value

9. Symbol defining statements
Symbols may be defined in assembly language to make programs more readable and easy to change (similar to named constants)
Common operators used
EQU =
Basic form
symbol EQU value or symbol = value
Example
+LDT # or RECLEN EQU
+LDT #RECLEN

10. ORG directive Used to indirectly assign values to symbols
format: ORG value
value is a constant or an expression
When ORG is encountered, the assembler resets the LOCCTR to the specified value
can be useful in label definition
altering LOCCTR may result in an incorrect assembly
If the assembler can remember the previous value of LOCCTR, an ORG statement with no value specified resets LOCCTR to previous value

11. ORG example
A program defines a table of 100 entries each having the structure: SYMBOL (6 bytes), VALUE (1 word), FLAGS (2 bytes)
STAB RESB 1100
If we want to refer to the individual fields
SYMBOL EQU STAB
VALUE EQU STAB+6
FLAGS EQU STAB+9
For an entry access
place in X the offset of the entry from the beginning of STAB, then fetch the field
e.g., LDA VALUE,X

12. Example, cont’d The structure of the table is not clear!
Using ORG instead
STAB RESB 1100
ORG STAB
SYMBOL RESB 6
VALUE RESW 1
FLAGS RESB 2
ORG STAB+1100
the last ORG statement is very important!

13. Restrictions
Standard two pass assembler requires all symbols to be defined in pass 1
all symbols on right of EQU or ORG must have been previously defined
error if not
The following sequence cannot be resolved
BETA EQU ALPHA
ALPHA EQU 5

14. Expressions May appear wherever a single operand is permitted
Formed using
operators +, -, *, / (integer operations only)
constants, symbols, special terms
May be either absolute or relative
constants are absolute
labels and PC references are relative
relative terms may not be used with * or /
a flag should be kept with each symbol in SYMTAB to indicate relative or absolute

15. Absolute and Relative Expressions
Absolute expressions
contains only absolute terms
if relative terms appear, they must occur in pairs of opposite signs (must subtract one from the other)
Relative expressions
all relative terms but one occur in pairs of opposite sign
the unpaired term must be of positive sign
cannot have * or /
Expressions which do not meet the conditions are flagged by the assembler as errors