- Subroutine Linkage

• JSUB (Jumps and places in register L)

• RSUB (Returns by jumping to the address contained in register L)
Basic Assembler Functions
Role of Assembler
Source Object
Assembler Code Linker
Program

Executable
Code

Loader
Outline
Basic Assembler Functions
Machine-dependent Assembler Features
Machine-independent Assembler Features
Assembler Design Options
Introduction to Assemblers
Fundamental functions
translating mnemonic operation codes to their machine
language equivalents
assigning machine addresses to symbolic labels

Machine dependency
different machine instruction formats and codes
Example Program
Purpose
reads records from input device (code F1)
copies them to output device (code 05)
at the end of the file, writes EOF on the output
device, then RSUB to the operating system
program
Assembler’s functions
Convert mnemonic operation codes to their
machine language equivalents
Convert symbolic operands to their equivalent
machine addresses 
Build the machine instructions in the proper
format
Convert the data constants to internal machine
representations
Write the object program and the assembly listing
Example of Instruction Assemble
STCH BUFFER,X 549039

8 1 15
opcode x address
m
(54)16 1 (001)2 (039)16

Forward reference
Difficulties: Forward Reference
Forward reference: reference to a label that is defined
later in the program.

Loc Label Operator Operand

1000 FIRST STL RETADR

1003 CLOOP JSUB RDREC

… … … … …
1012 J CLOOP
… … … … …
1033 RETADR RESW 1
Two Pass Assembler
Pass 1
 Assign addresses to all statements in the program
 Save the values assigned to all labels for use in Pass 2
 Perform some processing of assembler directives
Pass 2
 Assemble instructions
 Generate data values defined by BYTE, WORD
 Perform processing of assembler directives not done in Pass 1
 Write the object program and the assembly listing
Two Pass Assembler
Read from input line
LABEL, OPCODE, OPERAND

Source
program

Intermediate Object
Pass 1 Pass 2
file codes

OPTAB SYMTAB SYMTAB

Search OPTAB for OPCODE
Example Program
Data transfer (RD, WD)
a buffer is used to store record
buffering is necessary for different I/O rates
the end of each record is marked with a null
character (0016)
the end of the file is indicated by a zero-length
record
Subroutines (JSUB, RSUB)
RDREC, WRREC
save link register first before nested jump
Assembler Directives
Pseudo-Instructions
Not translated into machine instructions
Providing information to the assembler
Basic assembler directives
START
END
BYTE
WORD
RESB
RESW
Object Program
Header
Col. 1 H
Col. 2~7 Program name
Col. 8~13 Starting address (hex)
Col. 14-19 Length of object program in bytes (hex)
Text
Col.1 T
Col.2~7 Starting address in this record (hex)
Col. 8~9 Length of object code in this record in bytes (hex)
Col. 10~69 Object code (69-10+1)/6=10 instructions
End
Col.1 E
Col.2~7 Address of first executable instruction (hex)
(END program_name)
Object Code
H COPY 001000 00107A
T 001000 1E 141033 482039 001036 281030 301015 482061 ...
T 00101E 15 0C1036 482061 081044 4C0000 454F46 000003 000000
T 002039 1E 041030 001030 E0205D 30203F D8205D 281030 …
T 002057 1C 101036 4C0000 F1 001000 041030 E02079 302064 …
T 002073 07 382064 4C0000 05
E 001000
Object Code
Data Structures
Operation Code Table (OPTAB)
Symbol Table (SYMTAB)
Location Counter(LOCCTR)
OPTAB (operation code table)
Content
menmonic, machine code (instruction format, length)
etc.
Characteristic
static table
Implementation
array or hash table, easy for search
SYMTAB (symbol table)
COPY 1000
Content FIRST 1000
CLOOP 1003
label name, value, flag, (type, length) ENDFIL1015
etc.
Characteristic EOF 1024
THREE 102D
dynamic table (insert, delete, search) ZERO 1030
RETADR 1033
Implementation
LENGTH 1036
hash table, non-random keys, hashing function
BUFFER 1039
RDREC 2039
Example
SUM START 4000
FIRST LDX ZERO
LDA ZERO
LOOP ADD TABLE,X
TIX COUNT
JLT LOOP
STA TOTAL
RSUB
TABLE RESW 2000
COUNT RESW 1
ZERO WORD 0
TOTAL RESW 1
END FIRST
Assembler Design
Machine Dependent Assembler Features
 instruction formats and addressing modes
 program relocation
Machine Independent Assembler Features
 literals
 symbol-defining statements
 expressions
 program blocks
 control sections and program linking
Instruction formats and addressing modes
Program relocation
Instruction Format and Addressing Mode
SIC/XE
 PC-relative or Base-relative addressing: op m
 Indirect addressing: op @m
 Immediate addressing: op #c
 Extended format: +op m
 Index addressing: op m,x
 register-to-register instructions
 larger memory -> multi-programming (program allocation)
Translation
Register translation
 register name (A, X, L, B, S, T, F, PC, SW) and their
values (0,1, 2, 3, 4, 5, 6, 8, 9)
 preloaded in SYMTAB
Address translation
 Most register-memory instructions use program counter
relative or base relative addressing
 Format 3: 12-bit address field
 base-relative: 0~4095
 pc-relative: -2048~2047

 Format 4: 20-bit address field

PC-Relative Addressing Modes
PC-relative
 10 0000 FIRST STL RETADR 17202D

op(6) n I xbp e disp(12)

(14)16 1 1 0 0 1 0 (02D) 16
 displacement= RETADR - PC = 30-3 = 2D
 40 0017 J CLOOP 3F2FEC

op(6) n I xbp e disp(12)

(3C)16 110010 (FEC) 16
 displacement= CLOOP-PC= 6 - 1A= -14= FEC
Base-Relative Addressing Modes
Base-relative
 base register is under the control of the programmer
 12 LDB #LENGTH
 13 BASE LENGTH
 160 104E STCH BUFFER, X 57C003

( 54 )16op(6)1 1 1 1 0n0 I x b p (e003 ) 16 disp(12)

(54) 111010 0036-1051= -101B16
 displacement= BUFFER - B = 0036 - 0033 = 3
 NOBASE is used to inform the assembler that the contents of the base
register no longer be relied upon for addressing
Immediate

Address Translation
Immediate addressing
 55 0020 LDA #3 010003
op(6) n I xbp e disp(12)
( 00 )16 010000 ( 003 ) 16

 133 103C +LDT #4096 75101000

op(6) n I xbp e disp(20)
( 74 )16 010001 ( 01000 ) 16
Immediate Address Translation (Cont.)
Immediate addressing
 12 0003 LDB #LENGTH 69202D
op(6) n I xbp e disp(12)
( 68)16 010010 ( 02D ) 16
( 68)16 010000 ( 033)16 690033

 the immediate operand is the symbol LENGTH

 the address of this symbol LENGTH is loaded into register B

 LENGTH=0033=PC+displacement=0006+02D

 if immediate mode is specified, the target address becomes the

operand
Indirect Address Translation
Indirect addressing
target addressing is computed as usual (PC-relative or
BASE-relative)
only the n bit is set to 1
 70 002A J @RETADR 3E2003
op(6) n I xbp e disp(12)

( 3C )16 100010 ( 003 ) 16

 TA=RETADR=0030
 TA=(PC)+disp=002D+0003
Program Relocation
Example Fig. 2.1
Absolute program, starting address 1000
e.g. 55 101B LDA THREE 00102D
Relocate the program to 2000
e.g. 55 101B LDA THREE 00202D
Each Absolute address should be modified
Example Fig. 2.5:
 Except for absolute address, the rest of the instructions need not be
modified
 not a memory address (immediate addressing)
 PC-relative, Base-relative

 The only parts of the program that require modification at load time are
those that specify direct addresses
Example
Relocatable Program
Modification record
Col 1 M
Col 2-7 Starting location of the address field to be
modified, relative to the beginning of the
program
Col 8-9 length of the address field to be modified, in half-
bytes
Literals
Design idea
Let programmers to be able to write the value of a
constant operand as a part of the instruction that uses
it.
This avoids having to define the constant elsewhere in
the program and make up a label for it.
Example
 e.g. 45 001A ENDFIL LDA =C’EOF’ 032010
 93 LTORG
 002D * =C’EOF’ 454F46
 e.g. 215 1062 WLOOP TD =X’05’ E32011
Chap 2
Literals vs. Immediate Operands
Immediate Operands
The operand value is assembled as part of the machine
instruction
 e.g. 55 0020 LDA #3 010003
Literals
The assembler generates the specified value as a
constant at some other memory location
 e.g. 45 001A ENDFILLDA =C’EOF’032010
Compare (Fig. 2.6)
 e.g. 45 001A ENDFIL LDA EOF 032010
 80 002D EOF BYTE C’EOF’ 454F46
Literal - Implementation (1/3)
Literal pools
Normally literals are placed into a pool at the end of the
program
 see Fig. 2.10 (END statement)
In some cases, it is desirable to place literals into a pool
at some other location in the object program
 assembler directive LTORG
 reason: keep the literal operand close to the instruction
Literal - Implementation (2/3)
Duplicate literals
e.g. 215 1062 WLOOP TD =X’05’
e.g. 230 106B WD =X’05’
The assemblers should recognize duplicate literals and
store only one copy of the specified data value
 Comparison of the defining expression
 Same literal name with different value, e.g. LOCCTR=*

 Comparison of the generated data value

 The benefits of using generate data value are usually not great

enough to justify the additional complexity in the assembler

Literal - Implementation (3/3)
LITTAB
 literal name, the operand value and length, the address assigned to the
operand
Pass 1
 build LITTAB with literal name, operand value and length, leaving the
address unassigned
 when LTORG statement is encountered, assign an address to each
literal not yet assigned an address
Pass 2
 search LITTAB for each literal operand encountered
 generate data values using BYTE or WORD statements
 generate modification record for literals that represent an address in the
program
Symbol-Defining Statements
Labels on instructions or data areas
the value of such a label is the address assigned to the
statement
Defining symbols
symbol EQU value
value can be:  constant,  other symbol, 
expression
making the source program easier to understand
no forward reference
Symbol-Defining Statements
Example 1
 MAXLEN +LDT
EQU 4096 #4096
 +LDT #MAXLEN
Example 2 (Many general purpose registers)
 BASE EQU R1
 COUNT EQU R2
 INDEX EQU R3
Example 3
 MAXLEN EQU BUFEND-BUFFER
ORG (origin)
Indirectly assign values to symbols
Reset the location counter to the specified value
 ORG value
Value can be:  constant,  other symbol,  expression
No forward reference
Example
SYMBOL: 6bytes
SYMBOL VALUE FLAGS
VALUE: 1word STAB
(100 entries)
FLAGS: 2bytes
 LDA VALUE, X . . .
. . .
. . .
ORG Example
Using EQU statements
 STAB RESB 1100
 SYMBOL EQU STAB
 VALUE EQU STAB+6
 FLAG EQU STAB+9
Using ORG statements
 STAB RESB 1100
 ORG STAB
 SYMBOL RESB 6
 VALUE RESW 1
 FLAGS RESB 2
 ORG STAB+1100
Expressions
Expressions can be classified as absolute expressions or
relative expressions
 MAXLEN EQU BUFEND-BUFFER
 BUFEND and BUFFER both are relative terms, representing
addresses within the program
 However the expression BUFEND-BUFFER represents an
absolute value
When relative terms are paired with opposite signs, the
dependency on the program starting address is canceled
out; the result is an absolute value
SYMTAB
None of the relative terms may enter into a multiplication
or division operation
Errors:
 BUFEND+BUFFER
 100-BUFFER
 3*BUFFER
The type of an expression
keep track of the types of all symbols defined in the
program Symbol Type Value
RETADR R 30
BUFFER R 36
BUFEND R 1036
MAXLEN A 1000
Example 2.9
SYMTAB Name Value LITTAB
COPY 0 C'EOF' 454F46 3 002D
FIRST 0 X'05' 05 1 1076
CLOOP 6
ENDFIL 1A
RETADR 30
LENGTH 33
BUFFER 36
BUFEND 1036
MAXLEN 1000
RDREC 1036
RLOOP 1040
EXIT 1056
INPUT 105C
WREC 105D
WLOOP 1062
Program Blocks
Program blocks
refer to segments of code that are rearranged within a
single object program unit
USE [blockname]
Default block
Each program block may actually contain several
separate segments of the source program
Program Blocks - Implementation
Pass 1
 each program block has a separate location counter
 each label is assigned an address that is relative to the
start of the block that contains it
 at the end of Pass 1, the latest value of the location
counter for each block indicates the length of that block
 the assembler can then assign to each block a starting
address in the object program
Pass 2
 The address of each symbol can be computed by adding
the assigned block starting address and the relative
address of the symbol to that block
Example
Each source line is given a relative address assigned and a
block number
Block name Block number Address Length
(default) 0 0000 0066
CDATA 1 0066 000B
CBLKS 2 0071 1000
For absolute symbol, there is no block number
line 107
Example
 20 0006 0 LDA LENGTH 032060
 LENGTH=(Block 1)+0003= 0066+0003= 0069
 LOCCTR=(Block 0)+0009= 0009
Program Readability
Program readability
 No extended format instructions on lines 15, 35, 65
 No needs for base relative addressing (line 13, 14)
 LTORG is used to make sure the literals are placed ahead of
any large data areas (line 253)
Object code
It is not necessary to physically rearrange the generated
code in the object program
Control Sections and Program Linking
Control Sections
are most often used for subroutines or other logical
subdivisions of a program
the programmer can assemble, load, and manipulate
each of these control sections separately
instruction in one control section may need to refer to
instructions or data located in another section
because of this, there should be some means for linking
control sections together
External Definition and References
External definition
EXTDEF name [, name]
EXTDEF names symbols that are defined in this control
section and may be used by other sections
External reference
EXTREF name [,name]
EXTREF names symbols that are used in this control
section and are defined elsewhere
Example
 15 0003 CLOOP +JSUB RDREC 4B100000
 160 0017 +STCH BUFFER,X
57900000
 190 0028 MAXLEN WORD BUFEND-BUFFER 000000
 Implementation
The assembler must include information in the object program that
will cause the loader to insert proper values where they are required
Define record
 Col. 1 D
 Col. 2-7 Name of external symbol defined in this control section
 Col. 8-13 Relative address within this control section (hexadeccimal)
 Col.14-73 Repeat information in Col. 2-13 for other external symbols
Refer record
 Col. 1 R
 Col. 2-7 Name of external symbol referred to in this control section
 Col. 8-73 Name of other external reference symbols
Modification Record
Modification record
 Col. 1 M
 Col. 2-7 Starting address of the field to be modified
(hexiadecimal)
 Col. 8-9 Length of the field to be modified, in half-bytes
(hexadeccimal)
 Col.11-16 External symbol whose value is to be added to or
subtracted from the indicated field
 Note: control section name is automatically an external symbol,
i.e. it is available for use in Modification records.
Example
 M00000405+RDREC
 M00000705+COPY
External References in Expression
Earlier definitions
required all of the relative terms be paired in an expression (an
absolute expression), or that all except one be paired (a relative
expression)
New restriction
Both terms in each pair must be relative within the same
control section
 Ex: BUFEND-BUFFER
 Ex: RDREC-COPY
In general, the assembler cannot determine whether or not
the expression is legal at assembly time. This work will be
handled by a linking loader.
 One-pass assemblers
Multi-pass assemblers
Two-pass assembler with overlay structure
Two-Pass Assembler with Overlay Structure
For small memory
pass 1 and pass 2 are never required at the same time
three segments
 root: driver program and shared tables and subroutines
 pass 1

 pass 2

tree structure
overlay program
One-Pass Assemblers
Main problem
forward references
 data items
 labels on instructions

Solution
data items: require all such areas be defined before they
are referenced
labels on instructions: no good solution

Chap 2
One-Pass Assemblers
Main Problem
forward reference
 data items
 labels on instructions

Two types of one-pass assembler

load-and-go
 produces object code directly in memory for immediate
execution
the other
 produces usual kind of object code for later execution
Load-and-go Assembler
Characteristics
Useful for program development and testing
Avoids the overhead of writing the object program out
and reading it back
Both one-pass and two-pass assemblers can be designed
as load-and-go.
However one-pass also avoids the over head of an
additional pass over the source program
For a load-and-go assembler, the actual address must
be known at assembly time, we can use an absolute
program
Forward Reference in One-pass Assembler
For any symbol that has not yet been defined
1. omit the address translation
2. insert the symbol into SYMTAB, and mark this symbol
undefined
3. the address that refers to the undefined symbol is
added to a list of forward references associated with the
symbol table entry
4. when the definition for a symbol is encountered, the
proper address for the symbol is then inserted into any
instructions previous generated according to the
forward reference list
Load-and-go Assembler (Cont.)
At the end of the program
any SYMTAB entries that are still marked with *
indicate undefined symbols
search SYMTAB for the symbol named in the END
statement and jump to this location to begin execution
The actual starting address must be specified at
assembly time
Example
Producing Object Code
When external working-storage devices are not available
or too slow (for the intermediate file between the two
passes
Solution:
 When definition of a symbol is encountered, the assembler
must generate another Tex record with the correct operand
address
 The loader is used to complete forward references that could
not be handled by the assembler
 The object program records must be kept in their original
order when they are presented to the loader
Example:
Multi-Pass Assemblers
Restriction on EQU and ORG
no forward reference, since symbols’ value can’t be
defined during the first pass
Example
Use link list to keep track of whose value depend on an
undefined symbol