Assembler (2)

Assemblers

System Software
by Leland L. Beck
Chapter 2

Role of Assembler

Source Object
Assembler Linker
Program Code

Executable
Code

Loader

Chap 2

Chapter 2 -- Outline
s Basic Assembler Functions
s Machine-dependent Assembler Features
s Machine-independent Assembler Features
s Assembler Design Options

Chap 2

Introduction to Assemblers
s Fundamental functions
x translating mnemonic operation codes to their
machine language equivalents
x assigning machine addresses to symbolic
labels

s Machine dependency
x different machine instruction formats and codes

Chap 2

Example Program (Fig. 2.1)

s Purpose
x reads records from input device (code F1)
x copies them to output device (code 05)
x at the end of the file, writes EOF on the output
device, then RSUB to the operating system
x program

Chap 2

Example Program (Fig. 2.1)
s Data transfer (RD, WD)
x a buffer is used to store record
x buffering is necessary for different I/O rates
x the end of each record is marked with a null
character (0016)
x the end of the file is indicated by a zero-length
record
s Subroutines (JSUB, RSUB)
x RDREC, WRREC
x save link register first before nested jump

Chap 2

Assembler Directives
s Pseudo-Instructions
x Not translated into machine instructions
x Providing information to the assembler
s Basic assembler directives
x START
x END
x BYTE
x WORD
x RESB
x RESW

Chap 2

Object Program
s 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)
s 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. 10~69Object code (69-10+1)/6=10 instructions
s End
Col.1 E
Col.2~7 Address of first executable instruction (hex)
(END program_name)
Chap 2

Fig. 2.3
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

Chap 2

Figure 2.1 (Pseudo code)
Program copy {
save return address;
cloop: call subroutine RDREC to read one record;
if length(record)=0 {
call subroutine WRREC to write EOF;
} else {
call subroutine WRREC to write one record;
goto cloop;
}
load return address
return to caller
}

Chap 2

An Example (Figure 2.1, Cont.)
EOR:
Subroutine RDREC { character x‘00’
clear A, X register to 0;
rloop: read character from input device to A register
if not EOR {
store character into buffer[X];
X++;
if X < maximum length
goto rloop;
}
store X to length(record);
return
}

Chap 2

An Example (Figure 2.1, Cont.)
Subroutine WDREC {
clear X register to 0;
wloop: get character from buffer[X]
write character from X to output device
X++;
if X < length(record)
goto wloop;
return
}

Chap 2

Assembler’s functions
s Convert mnemonic operation codes to
their machine language equivalents
s Convert symbolic operands to their
equivalent machine addresses 
s Build the machine instructions in the
proper format
s Convert the data constants to internal
machine representations
s Write the object program and the
assembly listing
Chap 2

Example of Instruction Assemble

STCH BUFFER,X 549039
8 1 15
opcode x address
m
(54)16 1 (001)2 (039)16

s Forward reference

Chap 2

Difficulties: Forward Reference
s 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

Chap 2

Two Pass Assembler
s Pass 1
x Assign addresses to all statements in the program
x Save the values assigned to all labels for use in Pass 2
x Perform some processing of assembler directives
s Pass 2
x Assemble instructions
x Generate data values defined by BYTE, WORD
x Perform processing of assembler directives not done in
Pass 1
x Write the object program and the assembly listing

Chap 2

Two Pass Assembler
s Read from input line
x LABEL, OPCODE, OPERAND

Source
program

Intermediate Object
Pass 1 Pass 2
file codes

OPTAB SYMTAB SYMTAB

Chap 2

Data Structures

s Operation Code Table (OPTAB)
s Symbol Table (SYMTAB)
s Location Counter(LOCCTR)

Chap 2

OPTAB (operation code table)
s Content
x menmonic, machine code (instruction format,
length) etc.
s Characteristic
x static table
s Implementation
x array or hash table, easy for search

Chap 2

SYMTAB (symbol table)
s Content COPY 1000
x label name, value, flag, (type, length) etc.1000
FIRST
CLOOP 1003
s Characteristic ENDFIL 1015
EOF 1024
x dynamic table (insert, delete, search)
THREE 102D
s Implementation ZERO 1030
RETADR 1033
x hash table, non-random keys, hashing function
LENGTH 1036
BUFFER 1039
RDREC 2039

Chap 2

Homework #3
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

Chap 2

Assembler Design
s Machine Dependent Assembler Features
x instruction formats and addressing modes
x program relocation
s Machine Independent Assembler Features
x literals
x symbol-defining statements
x expressions
x program blocks
x control sections and program linking

Chap 2

Machine-dependent
Assembler Features
Sec. 2-2
s Instruction formats and addressing modes
s Program relocation

Instruction Format and Addressing Mode

s SIC/XE
x PC-relative or Base-relative addressing: op m
x Indirect addressing: op @m
x Immediate addressing: op #c
x Extended format: +op m
x Index addressing: op m,x
x register-to-register instructions
x larger memory -> multi-programming (program allocation)
s Example program

Chap 2

Translation
s Register translation
x register name (A, X, L, B, S, T, F, PC, SW) and their
values (0,1, 2, 3, 4, 5, 6, 8, 9)
x preloaded in SYMTAB
s Address translation
x Most register-memory instructions use program
counter relative or base relative addressing
x Format 3: 12-bit address field
3 base-relative: 0~4095
3 pc-relative: -2048~2047
x Format 4: 20-bit address field

Chap 2

PC-Relative Addressing Modes

s PC-relative
x 10 0000 FIRST STL RETADR 17202D

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

(14)16 110010 (02D) 16
3 displacement= RETADR - PC = 30-3 = 2D
x 40 0017 J CLOOP 3F2FEC


(3C)16 110010 (FEC) 16
3 displacement= CLOOP-PC= 6 - 1A= -14= FEC
Chap 2

Base-Relative Addressing Modes
s Base-relative
x base register is under the control of the programmer
x 12 LDB #LENGTH
x 13 BASE LENGTH
x 160 104E STCH BUFFER, X 57C003

( 54 )16 111100 ( 003 ) 16
(54) 111010 0036-1051= -101B16
3 displacement= BUFFER - B = 0036 - 0033 = 3
x NOBASE is used to inform the assembler that the contents
of the base register no longer be relied upon for addressing
Chap 2

Immediate Address Translation

s Immediate addressing
x 55 0020 LDA #3 010003
( 00 )16 010000 ( 003 ) 16

x 133 103C +LDT #4096 75101000
( 74 )16 010001 ( 01000 ) 16

Chap 2

Immediate Address Translation (Cont.)

s Immediate addressing
x 12 0003 LDB #LENGTH 69202D
( 68)16 010010 ( 02D ) 16
( 68)16 010000 ( 033)16 690033

3 the immediate operand is the symbol LENGTH
3 the address of this symbol LENGTH is loaded into

register B
3 LENGTH=0033=PC+displacement=0006+02D

3 if immediate mode is specified, the target address

becomes the operand Chap 2

Indirect Address Translation
s Indirect addressing
x target addressing is computed as usual (PC-
relative or BASE-relative)
x only the n bit is set to 1
x 70 002A J @RETADR 3E2003


( 3C )16 100010 ( 003 ) 16
3 TA=RETADR=0030
3 TA=(PC)+disp=002D+0003

Chap 2

Program Relocation
s Example Fig. 2.1
x Absolute program, starting address 1000
e.g. 55 101B LDA THREE 00102D
x Relocate the program to 2000
e.g. 55 101B LDA THREE 00202D
x Each Absolute address should be modified
s Example Fig. 2.5:
x Except for absolute address, the rest of the instructions need
not be modified
3 not a memory address (immediate addressing)
3 PC-relative, Base-relative
x The only parts of the program that require modification at
load time are those that specify direct addresses Chap 2

Relocatable Program

s Modification record
x Col 1 M
x Col 2-7 Starting location of the address field to be
modified, relative to the beginning of the program
x Col 8-9 length of the address field to be modified, in half-
bytes

Chap 2

Machine-Independent Assembler
Features
Literals
Symbol Defining Statement
Expressions
Program Blocks
Control Sections and Program
Linking

Literals
s Design idea
x Let programmers to be able to write the value
of a constant operand as a part of the
instruction that uses it.
x This avoids having to define the constant
elsewhere in the program and make up a label
for it.
s Example
x e.g. 45 001A ENDFIL LDA =C’EOF’ 032010
x 93 LTORG
x 002D * =C’EOF’ 454F46
x e.g. 215 1062 WLOOP TD =X’05’ E32011
Chap 2

Literals vs. Immediate Operands
s Immediate Operands
x The operand value is assembled as part of the
machine instruction
x e.g. 55 0020 LDA #3 010003
s Literals
x The assembler generates the specified value
as a constant at some other memory location
x e.g. 45 001A ENDFILLDA =C’EOF’ 032010
s Compare (Fig. 2.6)
x e.g. 45 001A ENDFIL LDA EOF 032010
x 80 002D EOF BYTE C’EOF’454F46
Chap 2

Literal - Implementation (1/3)
s Literal pools
x Normally literals are placed into a pool at the
end of the program
3 see Fig. 2.10 (END statement)
x In some cases, it is desirable to place literals
into a pool at some other location in the object
program
3 assembler directive LTORG
3 reason: keep the literal operand close to the

instruction

Chap 2

s Duplicate literals
x e.g. 215 1062 WLOOP TD =X’05’
x e.g. 230 106B WD =X’05’
x The assemblers should recognize duplicate
literals and store only one copy of the specified
data value
3 Comparison of the defining expression
• Same literal name with different value, e.g.
LOCCTR=*
3 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
Chap 2

s LITTAB
x literal name, the operand value and length, the address
assigned to the operand
s Pass 1
x build LITTAB with literal name, operand value and length,
leaving the address unassigned
x when LTORG statement is encountered, assign an address to
each literal not yet assigned an address
s Pass 2
x search LITTAB for each literal operand encountered
x generate data values using BYTE or WORD statements
x generate modification record for literals that represent an
address in the program
Chap 2

Symbol-Defining Statements
s Labels on instructions or data areas
x the value of such a label is the address
assigned to the statement
s Defining symbols
x symbol EQU value
x value can be: x constant, y other symbol, z
expression
x making the source program easier to
understand
x no forward reference

Chap 2

Symbol-Defining Statements
s Example 1
x MAXLEN EQU 4096
x +LDT #MAXLEN +LDT #4096
s Example 2 (Many general purpose registers)
x BASE EQU R1
x COUNT EQU R2
x INDEX EQU R3
s Example 3
x MAXLEN EQU BUFEND-BUFFER

Chap 2

ORG (origin)
s Indirectly assign values to symbols
s Reset the location counter to the specified value
3 ORG value
s Value can be: x constant, y other symbol, z
expression
s No forward reference
s Example
x SYMBOL: 6bytes
SYMBOL VALUE FLAGS
x VALUE: 1word STAB
x FLAGS: 2bytes (100 entries)
x LDA VALUE, X
. . .
. . .
. . .
Chap 2

ORG Example
s Using EQU statements
x STAB RESB 1100
x SYMBOL EQU STAB
x VALUE EQU STAB+6
x FLAG EQU STAB+9
s Using ORG statements
x STAB RESB 1100
x ORG STAB
x SYMBOL RESB 6
x VALUE RESW 1
x FLAGS RESB 2
x ORG STAB+1100
Chap 2

Expressions
s Expressions can be classified as absolute
expressions or relative expressions
x MAXLEN EQU BUFEND-BUFFER
x BUFEND and BUFFER both are relative terms,
representing addresses within the program
x However the expression BUFEND-BUFFER represents
an absolute value
s When relative terms are paired with opposite
signs, the dependency on the program starting
address is canceled out; the result is an absolute
value

Chap 2

SYMTAB
s None of the relative terms may enter into a
multiplication or division operation
s Errors:
x BUFEND+BUFFER
x 100-BUFFER
x 3*BUFFER
s The type of an expression
x 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
Chap 2

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
Chap 2

Program Blocks
s Program blocks
x refer to segments of code that are rearranged
within a single object program unit
x USE [blockname]
x Default block
x Example: Figure 2.11
x Each program block may actually contain
several separate segments of the source
program

Chap 2

Program Blocks - Implementation
s Pass 1
x each program block has a separate location counter
x each label is assigned an address that is relative to the
start of the block that contains it
x at the end of Pass 1, the latest value of the location
counter for each block indicates the length of that block
x the assembler can then assign to each block a starting
address in the object program
s Pass 2
x 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

Chap 2

Figure 2.12
s 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

s For absolute symbol, there is no block number
x line 107
s Example
x 20 0006 0 LDA LENGTH 032060
x LENGTH=(Block 1)+0003= 0066+0003= 0069
x LOCCTR=(Block 0)+0009= 0009
Chap 2

Program Readability
s Program readability
x No extended format instructions on lines 15, 35, 65
x No needs for base relative addressing (line 13, 14)
x LTORG is used to make sure the literals are placed
ahead of any large data areas (line 253)
s Object code
x It is not necessary to physically rearrange the
generated code in the object program
x see Fig. 2.13, Fig. 2.14

Chap 2

Control Sections and Program Linking
s Control Sections
x are most often used for subroutines or other
logical subdivisions of a program
x the programmer can assemble, load, and
manipulate each of these control sections
separately
x instruction in one control section may need to
refer to instructions or data located in another
section
x because of this, there should be some means
for linking control sections together
x Fig. 2.15, 2.16
Chap 2

External Definition and References
s External definition
x EXTDEF name [, name]
x EXTDEF names symbols that are defined in this
control section and may be used by other sections
s External reference
x EXTREF name [,name]
x EXTREF names symbols that are used in this
control section and are defined elsewhere
s Example
x 15 0003 CLOOP +JSUB RDREC 4B100000
x 160 0017 +STCH BUFFER,X 57900000
x 190 0028 MAXLEN WORD BUFEND-BUFFER 0000002
Chap

Implementation
s The assembler must include information in the object
program that will cause the loader to insert proper values
where they are required
s Define record
x Col. 1 D
x Col. 2-7 Name of external symbol defined in this control section
x Col. 8-13 Relative address within this control section (hexadeccimal)
x Col.14-73 Repeat information in Col. 2-13 for other external symbols
s Refer record
x Col. 1 D
x Col. 2-7 Name of external symbol referred to in this control section
x Col. 8-73 Name of other external reference symbols

Chap 2

Modification Record
s Modification record
x Col. 1 M
x Col. 2-7 Starting address of the field to be modified
(hexiadecimal)
x Col. 8-9 Length of the field to be modified, in half-bytes
(hexadeccimal)
x Col.11-16 External symbol whose value is to be added to or
subtracted from the indicated field
x Note: control section name is automatically an external symbol,
i.e. it is available for use in Modification records.
s Example
x Figure 2.17
x M00000405+RDREC
x M00000705+COPY Chap 2

External References in Expression
s Earlier definitions
x required all of the relative terms be paired in an
expression (an absolute expression), or that all
except one be paired (a relative expression)
s New restriction
x Both terms in each pair must be relative within
the same control section
x Ex: BUFEND-BUFFER
x Ex: RDREC-COPY
s
In general, the assembler cannot determine
whether or not the expression is legal at
assembly time. This work will be handled by a
Chap 2
linking loader.

Assembler Design Options

One-pass assemblers
Multi-pass assemblers
Two-pass assembler with overlay
structure

Two-Pass Assembler with Overlay
Structure
s For small memory
x pass 1 and pass 2 are never required at the
same time
x three segments
3 root: driver program and shared tables and
subroutines
3 pass 1

3 pass 2

x tree structure
x overlay program

Chap 2

One-Pass Assemblers
s Main problem
x forward references
3 data items
3 labels on instructions

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

Chap 2

One-Pass Assemblers
s Main Problem
x forward reference
3 data items
3 labels on instructions

s Two types of one-pass assembler
x load-and-go
3 produces object code directly in memory for
immediate execution
x the other
3 produces usual kind of object code for later
execution
Chap 2

Load-and-go Assembler
s Characteristics
x Useful for program development and testing
x Avoids the overhead of writing the object
program out and reading it back
x Both one-pass and two-pass assemblers can
be designed as load-and-go.
x However one-pass also avoids the over head
of an additional pass over the source program
x For a load-and-go assembler, the actual
address must be known at assembly time, we
can use an absolute program
Chap 2

Forward Reference in One-pass Assembler
s 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
Chap 2

Load-and-go Assembler (Cont.)
s At the end of the program
x any SYMTAB entries that are still marked with *
indicate undefined symbols
x search SYMTAB for the symbol named in the
END statement and jump to this location to
begin execution
s The actual starting address must be
specified at assembly time
s Example
x Figure 2.18, 2.19

Chap 2

Producing Object Code
s When external working-storage devices are not
available or too slow (for the intermediate file
between the two passes
s Solution:
x When definition of a symbol is encountered, the assembler
must generate another Tex record with the correct
operand address
x The loader is used to complete forward references that
could not be handled by the assembler
x The object program records must be kept in their original
order when they are presented to the loader
s Example: Figure 2.20

Chap 2

Multi-Pass Assemblers
s Restriction on EQU and ORG
x no forward reference, since symbols’ value
can’t be defined during the first pass
s Example
x Use link list to keep track of whose value
depend on an undefined symbol
s Figure 2.21

Chap 2

Assembler (2)

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