Dr. Ali Orooji

1
COP 3402 Systems Programming

• Dr. Ali Orooji
• School of EECSUniversity of Central Florida

2
Section 2.3.2

• Symbol-Defining Statements
• symbol EQU value
• assembler puts the symbol and value in
  symbol table
• 1000 L1 EQU 500
• L2
• L2-L3

3
Section 2.3.3

• Expressions
• assemblers generally allow , -, , /
• Absolute expression
• either only absolute terms
• or if relative terms, they occur in pairs
  with
• opposite signs
• relative terms may not enter into a
  multiplication or division
• operation
• Relative expression
• relative terms occur in pairs as above
  one extra
• relative term (positive sign)

4
Section 2.3.4

• Program Blocks
• segments of code that are logically rearranged
  within a single object program unit

5

• Pass 1
• - a separate location counter for each block
• - when storing a label in symbol table, store
  the
• block number as well
• At the end of pass 1, the length of each block is
  known
• the assembler can now assign a starting address
  to each
• block

Pass 2 the value for a label is the value in
the symbol table plus the starting address
for the block
6

• Note that the assembler does not physically
  rearrange the generated code in the object
  program (the assembler puts the load address with
  object code loader will load accordingly)

7
Section 2.3.5 Control Sections
- segments that are translated into independent
object program units
CSECT 1
CSECT 2
- need to link together different control
sections . assembler cannot do the
linking . assembler provides information
for each external reference that will
allow the loader to perform the
required linking
- need extended format (type 4) for instructions
containing external references
8
(No Transcript)
9
Two-Pass Assembler with Overlay Structure
main segment 20K Pass 1 segment
50K Pass 2 segment 60K
10
Section 2.4.1 One-Pass Assemblers Why did
we need two passes? forward references
- data items - labels Load-and-go
one-pass assemblers generate object code in
memory for immediate execution (no object
program is written out, and no loader is
needed)
11
. . . 1000 J RDREC
. . 101D J RDREC . .
2000 RDREC ...
12

• If the one-pass assembler generates object
  program
• on secondary storage, i.e., not load-and-go
  assembler
• - put zero when address not known (and
  remember the references using symtbl as before)
• - when address is known, generate records to
  fill references
• 1000 J RDREC 1000 J
  0000
• .
• .
• 101D J RDREC 101D J 0000
• .
• .
• 2000 RDREC
• 1000
  2000
• 
  101D 2000

13
Section 2.4.2

• Multi-Pass Assemblers
• in the two-pass assembler
• symbol defining directives
• MAXLEN EQU BUFEND-BUFFER
• symbols on the right hand side
  must be
• defined previously
• A EQU B
• B EQU C
• C EQU

14
HALFSZ EQU MAXLEN/2
15
MAXLEN EQU BUFEND-BUFFER
symtbl
of undef sym
ref ptr
name
value
expression
HALFSZ
1
MAXLEN / 2
MAXLEN
HALFSZ
2
BUFEND-BUFFER
BUFEND
MAXLEN
BUFFER
MAXLEN
PREVBT EQU BUFFER-1