1. Assemblers
CSCI/CMPE 3334
David Egle

2. Fundamental functions
Take a program written in assembly language and translate it to machine language (binary)
What does this entail?
Translate mnemonics to machine code
Convert symbolic operands to addresses
Convert constants into internal representation
Process assembler directives
Build machine language instruction
Write the object and listing files

3. Machine dependencies
The design and many features of assembler depend on underlying hardware
instruction set
instruction format
registers
memory management
Some features do not depend on hardware
Programmer conveniences
Program format
etc.

4. SIC assembly language General format
label – must begin in first column
operation – may be mnemonic or directive
operand – symbol, value, or hexadecimal address
comment
This is typical of many assembly languages. The specific formats may vary with respect to punctuation, etc.

5. SIC sample program (1/3)
COPY START 1000 COPY FILE FROM INPUT TO OUTPUT FIRST STL RETADR SAVE RETURN ADDRESS CLOOP JSUB RDREC READ INPUT RECORD LDA LENGTH TEST FOR EOF (LENGTH = 0) COMP ZERO JEQ ENDFIL EXIT IF EOF FOUND JSUB WRREC WRITE OUTPUT RECORD J CLOOP LOOP ENDFIL LDA EOF INSERT END OF FILE MARKER STA BUFFER LDA THREE SET LENGTH = 3 STA LENGTH JSUB WRREC WRITE EOF LDL RETADR GET RETURN ADDRESS RSUB RETURN TO CALLER EOF BYTE C'EOF' THREE WORD 3 ZERO WORD 0 RETADR RESW 1 LENGTH RESW 1 LENGTH OF RECORD BUFFER RESB BYTE BUFFER AREA

6. SIC sample program (2/3)
. . SUBROUTINE TO READ RECORD INTO BUFFER RDREC LDX ZERO CLEAR LOOP COUNTER LDA ZERO CLEAR A TO ZERO RLOOP TD INPUT TEST INPUT DEVICE JEQ RLOOP LOOP UNTIL READY RD INPUT READ CHARACTER INTO REGISTAR A COMP ZERO TEST FOR END OF RECORD (X'00') JEQ EXIT EXIT LOOP IF EOF STCH BUFFER,X STORE CHARCTER IN BUFFER TIX MAXLEN LOOP UNLESS MAX LENGTH JLT RLOOP HAS BEEN REACHED EXIT STX LENGTH SAVE RECORD LENGTH RSUB RETURN TO CALLER INPUT BYTE X'F1' CODE FOR INPUT DEVICE MAXLEN WORD 4096

7. SIC sample program (3/3)
. . SUBROUTINE TO WRITE RECORD FROM BUFFER WRREC LDX ZERO CLEAR LOOP COUNTER WLOOP TD OUTPUT TEST OUTPUT DEVICE JEQ WLOOP LOOP UNTIL READY LDCH BUFFER,X GET CHARCTER FROM BUFFER WD OUTPUT WRITE CHARACTER TIX LENGTH LOOP UNTIL ALL CHARACTERS JLT WLOOP HAVE BEEN WRITTEN RSUB RETURN TO CALLER OUTPUT BYTE X'05' CODE FOR OUTPUT DEVICE END FIRST

8. SIC assembly language (cont’d)
Some specifics regarding SIC
Directives (pseudo-ops)
START
END
BYTE
WORD
RESB
RESW
Indexing
uses “,X” format, as in STCH BUFFER,X
Comment lines
begin with period

9. End result – the object file
For ease in displaying the file, it is stored in ASCII with hexadecimal values for the code
NOTE: This is not the usual way of storing the object file
HCOPY__ A
T E C A0C D
T00101E150C C F
T E E0205D30203FD8205D C205E38203F
T C C0000F E DC20792C1036
T C000005
E001000

10. The process So, how do we get from the source FIRST STL RETADR
to the object code
141033
The STL part is easy – the code is 14 (hex)
BUT, what about the operand, RETADR?
What is its value? (It is called a forward reference)
Some preliminary work must be done!

11. Handling forward references
We need to know the address of RETADR before we can complete the instruction
Two possibilities
Store the partial instruction until the address is determined (one pass assembly)
Go through the code and determine ALL the addresses before starting the actual translation (two pass assembly)
Both have their advantages and disadvantages
We consider the two pass version first

12. Two pass assembler Pass one – BUILD SYMBOL TABLE
assign addresses to all statements
save addresses assigned to labels in symbol table
perform some processing of assembler directives
Pass two – TRANSLATION
convert mnemonics & symbols to machine code
convert constants
process remaining assembler directives
write machine code to object file; also write listing file
In both passes, try to detect any errors in syntax

13. Data structures The location counter (called LOCCTR in text) Tables
Initialized to address specified in START statement (which is a hexadecimal value)
As each statement is examined, it’s length is added to the LOCCTR
Tables
Table of opcode values (OPTAB)
Table of symbol values (SYMTAB)
Files
Intermediate file
Object file
Listing file

14. Opcode table Contains (for SIC) May also contain
mnemonic
its equivalent machine language value
May also contain
instruction format
required operands
Usually organized as a closed (static) hash table
fast retrieval with minimal searching

15. Symbol Table Contains (for SIC) May also contain
associated value (usually the address)
May also contain
type
length
scope information
Also organized as a hash table
but it is dynamic

16. Intermediate file Written in pass 1 and used in pass 2
Contains (at minimum)
source statement
address
mnemonic or its value
operand(s)
errors in statement
May contain pointers to OPTAB or SYMTAB

17. Object file Contains results of assembly
Format varies according to designer
Usually read by a linker/loader program
Several record types (this also can vary)
header record
linkage records
code records
end record

18. Listing file
Contains results of assembly for viewing by the programmer (or others)
Format varies but usually includes
address of statement
machine code equivalent
source statement (including comments)
errors associated with each statement

19. Algorithm for pass 1 (SIC)
read first input line and write to intermediate file
if OPCODE = ‘START’ then
{ save #{OPERAND} as starting address
initialize LOCCTR to starting address
read next input line and write to intermediate file }
else set LOCCTR=0
while OPCODE != ‘END’ do
{ if this is not a comment line then
handle line 1
save (LOCCTR – starting address) as program length

20. Pass 1: handle line 1 If there is a symbol in the LABEL field then
{ search SYMTAB for LABEL
if found then set error flag (duplicate label)
else insert (LABEL, LOCCTR) into SYMTAB }
search OPTAB for OPCODE
if found then add 3 to LOCCTR
else if OPCODE = ‘WORD’ then add 3 to LOCCTR
else if OPCODE = ‘RESB’ then add #[OPERAND] to LOCCTR
else if OPCODE = ‘RESW’ then add 3* #[OPERAND] to LOCCTR
else if OPCODE = ‘BYTE’ then
{ find length of constant in bytes
add length to LOCCTR }
else set error flag (invalid operation code)

21. Algorithm for pass 2 (SIC)
read first input line from intermediate file
if OPCODE = ‘START’ then
{ write listing line
read next input line }
write header record to object file
initialize first text record
while OPCODE != ‘END’ do
{ if this is not a comment line then
handle line 2
write listing line
write last text record to object file
write end record to object file
write last listing line

22. Pass 2: handle line 2 search OPTAB for OPCODE if found then
{ if there is a symbol in OPERAND field then
{ search SYMTAB for OPERAND
store symbol value as operand address
else
store 0 as operand address
set error flag (undefined symbol) }
else if OPCODE = ‘BYTE’ or ‘WORD’ then
convert constant to object code
if object code will not fit into current text record then
{ write text record to object file
initialize new text record }
add object code to text record

23. Example (Exercise 2.1.2) Assemble the following: SUM START 4000
FIRST LDX ZERO INITIALIZE REGISTERS
LDA ZERO
LOOP ADD TABLE,X ADD THE ELEMENTS
TIX COUNT ARE WE DONE?
JLT LOOP IF NOT, LOOP
STA TOTAL STORE THE TOTAL
RSUB AND RETURN
TABLE RESW FOR THE ARRAY
COUNT RESW 1 NUMBER OF ELEMENTS
ZERO WORD 0 CONSTANT
TOTAL RESW 1 PLACE FOR TOTAL
END FIRST