Computer Organization &
Computer Organization &
Assembly Languages Assembly Languages
Pu-Jen Cheng
Assembler
Adapted from the slides prepared by Beck for the book,
System Software: An Intro. to Systems Programming , 3rd Ed.
Overview
SIC Machine Architecture
SIC/XE Machine Architecture
Design and Implementation of Assembler
Source Object
Source
Program Assembler j Code
Loader Executable
Code Linker SIC, SIC/XE Program
Overview
SIC Machine Architecture
SIC/XE Machine Architecture
Design and Implementation of Assembler
Source Object
Source
Program Assembler j Code
Loader Executable
Code Linker SIC, SIC/XE Program
The Simplified Instructional Computer (SIC)
Memory
¾ Memory consists of 8-bit bytes
¾ Any 3 consecutive bytes form a word (24 bits)
¾ Total of 32768 (215) bytes in the computer memory
SIC Machine Architecture
o o 3 768 ( ) by es e co pu e e o y
Registers
¾ Five 24-bits registers
SIC Machine Architecture
Data Formats
¾ Integers are stored as 24-bit binary number
¾ 2’s complement representation for negative values
¾ Characters are stored using 8-bit ASCII codes
¾ No floating-point hardware on the standard version of SIC
Instruction Formats
¾ Standard version of SIC 8 1 15
opcode x address
Mode Indication Target address calculation
Direct x=0 TA=address
Indexed x=1 TA=address+(X)
(X): the contents of register X
SIC Machine Architecture
Instruction Set
¾ Load and store registers
LDA, LDX, STA, STX, etc.
¾ Integer arithmetic operations
ADD, SUB, MUL, DIV
All arithmetic operations involve register A and a word in memory, with the result being left in A
the result being left in A
¾ COMP
¾ Conditional jump instructions
JLT, JEQ, JGT
¾ Subroutine linkage
JSUB, RSUB
¾ I/O (transferring 1 byte at a time to/from the rightmost 8 bits of register A)
Test Device instruction (TD)
Read Data (RD)
Write Data (WD)
SIC Programming Example
LDA FIVE STA ALPHA LDCH CHARZ STCH C1
. . .
ALPHA RESW 1 one-word variable FIVE WORD 5 one-word constant CHARZ BYTE C’Z’ one-byte constant C1 RESB 1 one-byte variable
SIC Programming Example
LDX ZERO initialize index register to 0 MOVECH LDCH STR1,X load char from STR1 to reg A
STCH STR2,X
TIX ELEVEN add 1 to index, compare to 11 JLT MOVECH loop if “less than”
. . .
STR1 BYTE C’TEST STRING’
STR2 RESB 11 ZERO WORD 0 ELEVEN WORD 11
SIC Programming Example
LDA ZERO initialize index value to 0 STA INDEX
ADDLP LDX INDEX load index value to reg X
LDA ALPHA,X load word from ALPHA into reg A ADD BETA,X
STA GAMMA,X store the result in a word in GAMMA LDA INDEX
ADD THREE add 3 to index value STA INDEX
COMP K300 compare new index value to 300 JLT ADDLP loop if less than 300
...
...
INDEX RESW 1
ALPHA RESW 100 array variables—100 words each
BETA RESW 100
GAMMA RESW 100
ZERO WORD 0 one-word constants
THREE WORD 3
K300 WORD 300
SIC Programming Example
INLOOP TD INDEV test input device
JEQ INLOOP loop until device is ready (<) RD INDEV read one byte into register A STCH DATA
. .
OUTLP TD OUTDEV test output device
JEQ OUTLP loop until device is ready (<) LDCH DATA
WD OUTDEV write one byte to output device .
.
INDEV BYTE X’F1’ input device number OUTDEV BYTE X’05’ output device number DATA RESB 1
Overview
SIC Machine Architecture
SIC/XE Machine Architecture
Design and Implementation of Assembler
Source Object
Source
Program Assembler j Code
Loader Executable
Code Linker SIC, SIC/XE Program
SIC/XE Machine Architecture
An XE version (upward compatible)
Memory
¾ Maximum 1 megabyte (220 bytes)
Registers
¾ Additional registers are provided by SIC/XE
¾ Additional registers are provided by SIC/XE
Support 48-bit floating-point data type
SIC/XE Machine Architecture
Instruction Formats
8 op
8 4 4
op r1 r2
Format 1 (1 byte)
Format 2 (2 bytes)
Formats 1 and 2 are instructions that do not reference memory at all
6 1 1 1 1 1 1 12
op n i x b p e disp
Format 3 (3 bytes)
6 1 1 1 1 1 1 20
op n i x b p e address
Format 4 (4 bytes)
SIC/XE Machine Architecture
Addressing modes
¾ Base relative (n=1, i=1, b=1, p=0)
¾ Program-counter relative (n=1, i=1, b=0, p=1)
¾ Direct (n=1, i=1, b=0, p=0)
¾ Immediate (n=0, i=1, x=0)( , , )
¾ Indirect (n=1, i=0, x=0)
¾ Indexing (both n & i = 0 or 1, x=1)
¾ Extended (e=1 for format 4, e=0 for format 3)
SIC/XE Machine Architecture
Base Relative Addressing Mode
n i x b p e
opcode 1 1 1 0 disp
n=1 i=1 b=1 p=0 TA=(B)+disp (0≤disp ≤4095) n 1, i 1, b 1, p 0, TA (B)+disp (0≤disp ≤4095)
Program-Counter Relative Addressing Mode
n i x b p e
opcode 1 1 0 1 disp
n=1, i=1, b=0, p=1, TA=(PC)+disp (-2048≤disp ≤2047)
SIC/XE Machine Architecture
Direct Addressing Mode
n i x b p e
opcode 1 1 0 0 disp
n=1 i=1 b=0 p=0 TA=disp (0≤disp ≤4095) n 1, i 1, b 0, p 0, TA disp (0≤disp ≤4095)
n i x b p e
opcode 1 1 1 0 0 disp
n=1, i=1, b=0, p=0, TA=(X)+disp (with index addressing mode)
SIC/XE Machine Architecture
Immediate Addressing Mode
n i x b p e
opcode 0 1 0 disp
n=0, i=1, x=0, operand=disp, , , p p
Indirect Addressing Mode
n i x b p e
opcode 1 0 0 disp
n=1, i=0, x=0, TA=(disp)
SIC/XE Machine Architecture
Simple Addressing Mode
n i x b p e
opcode 0 0 disp
i=0 n=0 TA=bpe+disp (SIC standard) i 0, n 0, TA bpe+disp (SIC standard)
n i x b p e
opcode 1 1 disp
i=1, n=1, TA=disp (SIC/XE standard)
SIC/XE Machine Architecture
Instruction Format
SIC/XE Machine Architecture
Instruction Set
¾ Instructions to load and store the new registers
LDB, STB, etc.
¾ Floating-point arithmetic operations
ADDF, SUBF, MULF, DIVF Register move instruction
¾ Register move instruction
RMO
¾ Register-to-register arithmetic operations
ADDR, SUBR, MULR, DIVR
¾ Supervisor call instruction
SVC
¾ Input and Output
There are I/O channels that can be used to perform input and output while the CPU is executing other instructions
SIC/XE Programming Example
LDA #5
STA ALPHA LDCH #90 STCH C1 LDA FIVE
STA ALPHA LDCH CHARZ STCH C1
SIC version SIC/XE version
. . .
ALPHA RESW 1
C1 RESB 1
. . .
ALPHA RESW 1 FIVE WORD 5
CHARZ BYTE C’Z’
C1 RESB 1
SIC/XE Programming Example
LDS INCR LDA ALPHA
ADDR S,A A = A + S SUB #1 A = A - 1 STA BETA
LDA GAMMA ADDR S,A SUB #1 SUB #1
STA DELTA ...
...
ALPHA RESW 1 one-word variables BETA RESW 1
GAMMA RESW 1 DELTA RESW 1 INCR RESW 1
SIC/XE Programming Example
LDT #11 initialize register T to 11 LDX #0 initialize index register to 0 MOVECH LDCH STR1,X load char from STR1 to reg A
STCH STR2,X store char into STR2
TIXR T add 1 to index, compare to 11 JLT MOVECH loop if “less than” 11
. . .
STR1 BYTE C’TEST STRING’
STR2 RESB 11
SIC/XE Programming Example
LDS #3 LDT #300 LDX #0
ADDLP LDA ALPHA,X load from ALPHA to reg A ADD BETA,X
STA GAMMA,X store in a word in GAMMA ADDR S X add 3 to index value
ADDR S,X add 3 to index value COMPR X,T compare to 300
JLT ADDLP loop if less than 300 ...
...
ALPHA RESW 100 array variables—100 words each BETA RESW 100
GAMMA RESW 100
SIC/XE Programming Example
Overview
SIC Machine Architecture
SIC/XE Machine Architecture
Design and Implementation of Assembler
Source Object
Source
Program Assembler j Code
Loader Executable
Code Linker SIC, SIC/XE Program
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
Functions of a Basic Assembler
Mnemonic code (or instruction name) Æ opcode.
Symbolic operands (e.g., variable names) Æ addresses.
Choose the proper instruction format & addressing mode.
Constants Æ Numbers.
Output to object files and listing files.
Assembler Directives
Pseudo-Instructions
¾ Not translated into machine instructions
¾ Providing information to the assembler
Basic assembler directives
¾ START
¾ START
¾ END
¾ BYTE
¾ WORD
¾ RESB
¾ RESW
Example Program with Object Code
Line Loc Source statement Object code
5 1000 COPY START 1000
10 1000 FIRST STL RETADR 141033 15 1003 CLOOP JSUB RDREC 482039
20 1006 LDA LENGTH 001036
25 1009 COMP ZERO 281030
30 100C JEQ ENDFIL 301015
35 100F JSUB WRREC 482061
40 1012 J CLOOP 3C1003
45 1015 ENDFIL LDA EOF 00102A
45 1015 ENDFIL LDA EOF 00102A
50 1018 STA BUFFER 0C1039
55 101B LDA THREE 00102D
60 101E STA LENGTH 0C1036
65 1021 JSUB WRREC 482061
70 1024 LDL RETADR 081033
75 1027 RSUB 4C0000
80 102A EOF BYTE C’EOF’ 454F46
85 102D THREE WORD 3 000003
90 1030 ZERO WORD 0 000000
95 1033 RETADR RESW 1 100 1036 LENGTH RESW 1 105 1039 BUFFER RESB 4096
110 .
115 . SUBROUTINE TO READ RECORD INTO BUFFER
120 .
125 2039 RDREC LDX ZERO 041030
130 203C LDA ZERO 001030
135 203F RLOOP TD INPUT E0205D
140 2042 JEQ RLOOP 30203D
145 2045 RD INPUT D8205D
150 2048 COMP ZERO 281030
155 204B JEQ EXIT 302057
160 204E STCH BUFFER,X 549039
165 2051 TIX MAXLEN 2C205E
170 2054 JLT RLOOP 38203F
175 2057 EXIT STX LENGTH 101036
180 205A RSUB 4C0000
185 205D INPUT BYTE X’F1’ F1
190 205E MAXLEN WORD 4096 001000
195 .
200 . SUBROUTINE TO WRITE RECORD FROM BUFFER
205 .
210 2061 WRREC LDX ZERO 041030
215 2064 WLOOP TD OUTPUT E02079
220 2067 JEQ WLOOP 302064
225 206A LDCH BUFFER,X 509039
230 206D WD OUTPUT DC2079
235 2070 TIX LENGTH 2C1036
240 2073 JLT WLOOP 382064
245 2076 RSUB 4C0000
250 2079 OUTPUT BYTE X’05’ 05
255 END FIRST
Symbolic Operands
Mnemonic code (or instruction name) Æ opcode.
STL 1033 Æ opcode 14 10 33
U i bl i t d f dd
0001 0100 0 001 0000 0011 0011
Use variable names instead of memory addresses
¾ Labels (for jump instructions)
¾ Subroutines
¾ Constants COPY START 1000
…
LDA LEN
…
…
LEN RESW 1 forward references
Two Pass Assembler
Pass 1
¾ Assign addresses (LOC) to all statements in the program
¾ Save the values assigned to all labels for use in Pass 2
¾ Perform some processing of assembler directives
Pass 2
¾ Assemble instructions
¾ Generate data values defined by BYTE, WORD
¾ Perform processing of assembler directives not done in Pass 1
¾ Write the object program and the assembly listing
Two Pass Assembler
Pass 1 Intermediate Pass 2
file Object codes
Source program
file codes
OPTAB SYMTAB SYMTAB
Data Structures:
Operation Code Table (OPTAB) Symbol Table (SYMTAB)
Location Counter(LOCCTR)
Two Pass Assembler – Pass 1
Two Pass Assembler – Pass 2
OPTAB (operation code table)
Content
¾ Mnemonic, machine code (instruction format, length) etc.
Characteristic
¾ Static table
Implementation
¾ Array or hash table, easy for search
SYMTAB (symbol table)
Content
¾ Label name, value, flag, (type, length) etc.
Characteristic
¾ Dynamic table (insert, delete, search)
COPY 1000
FIRST 1000 CLOOP 1003 ENDFIL 1015
EOF 1024
THREE 102D
Implementation
¾ Hash table, non-random keys, hashing function
ZERO 1030
RETADR 1033 LENGTH 1036 BUFFER 1039 RDREC 2039
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
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)
Line Loc Source statement Object code
5 1000 COPY START 1000
10 1000 FIRST STL RETADR 141033 15 1003 CLOOP JSUB RDREC 482039
20 1006 LDA LENGTH 001036
25 1009 COMP ZERO 281030
30 100C JEQ ENDFIL 301015
35 100F JSUB WRREC 482061
40 1012 J CLOOP 3C1003
45 1015 ENDFIL LDA EOF 00102A
45 1015 ENDFIL LDA EOF 00102A
50 1018 STA BUFFER 0C1039
55 101B LDA THREE 00102D
60 101E STA LENGTH 0C1036
65 1021 JSUB WRREC 482061
70 1024 LDL RETADR 081033
75 1027 RSUB 4C0000
80 102A EOF BYTE C’EOF’ 454F46
85 102D THREE WORD 3 000003
90 1030 ZERO WORD 0 000000
95 1033 RETADR RESW 1 100 1036 LENGTH RESW 1 105 1039 BUFFER RESB 4096
Object Program Example
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 002057 1C 101036 4C0000 F1 001000 041030 E02079 302064 … T 002073 07 382064 4C0000 05
E 001000 Åstarting address
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
An SIC/XE Example
Line Loc Source statement Object code
5 0000 COPY START 0
10 0000 FIRST STL RETADR 17202D
12 0003 LDB #LENGTH 69202D
13 BASE LENGTH
15 0006 CLOOP +JSUB RDREC 4B101036
20 000A LDA LENGTH 032026
25 000D COMP #0 290000
30 0010 JEQ ENDFIL 332007
35 0013 JSUB WRREC 4B10105D
35 0013 +JSUB WRREC 4B10105D
40 0017 J CLOOP 3F2FEC
45 001A ENDFIL LDA EOF 032010
50 001D STA BUFFER 0F2016
55 0020 LDA #3 010003
60 0023 STA LENGTH 0F200D
65 0026 +JSUB WRREC 4B10105D
70 002A J @RETADR 3E2003
80 002D EOF BYTE C’EOF’ 454F46 95 0030 RETADR RESW 1
100 0033 LENGTH RESW 1 105 0036 BUFFER RESB 4096
115 . READ RECORD INTO BUFFER
120 .
125 1036 RDREC CLEAR X B410
130 1038 CLEAR A B400
132 103A CLEAR S B440
133 103C +LDT #4096 75101000
135 1040 RLOOP TD INPUT E32019
140 1043 JEQ RLOOP 332FFA
145 1046 RD INPUT DB2013
150 1049 COMPR A,S A004
155 104B JEQ EXIT 332008
160 104E STCH BUFFER,X 57C003
165 1051 TIXR T B850
170 1053 JLT RLOOP 3B2FEA
175 1056 EXIT STX LENGTH 134000
180 1059 RSUB 4F0000
185 105C INPUT BYTE X’F1’ F1
195 .
200 . WRITE RECORD FROM BUFFER
205 .
210 105D WRREC CLEAR X B410
212 105F LDT LENGTH 774000
215 1062 WLOOP TD OUTPUT E32011
220 1065 JEQ WLOOP 332FFA
225 1068 LDCH BUFFER,X 53C003
230 106B WD OUTPUT DF2008
235 106E TIXR T B850
...(omitted)
A Case of Object Code Generation
Line 10
STL RETADR Æ 17 20 2D
The mode bit p=1, meaning PC relative addressing mode.
OPCODE e Address
6 bits 12 bits
n i x b p
0001 01 1 1 0 0 1 0 0000 0010 1101
17 20 2D
Instruction Format and Addressing Mode
SIC/XE
¾ PC-relative or Base-relative addressing: op m
¾ Indirect addressing: op @m
¾ Immediate addressing: op #c
¾ Extended format: +op m
¾ Extended format: +op m
¾ Index addressing: op m,x
¾ register-to-register instructions
¾ larger memory -> multi-programming (program allocation)
Translation
Register translation
¾ Register name (A, X, L, B, S, T, F, PC, SW) and their values (0,1, 2, 3, 4, 5, 6, 8, 9)
¾ Preloaded in SYMTAB
Address translation
Address translation
¾ Most register-memory instructions use program counter relative or base relative addressing
¾ Format 3: 12-bit address field
Base-relative: 0~4095
PC-relative: -2048~2047
¾ Format 4: 20-bit address field
PC-Relative Addressing Mode
PC-relative
¾ 10 0000 FIRST STL RETADR 17202D
OPCODE n i x b p e Address
0001 01 1 1 0 0 1 0 (02D)
Displacement= RETADR - PC = 30-3 = 2D
¾ 40 0017 J CLOOP 3F2FEC
Displacement= CLOOP-PC= 6 - 1A= -14= FEC 0001 01 1 1 0 0 1 0 (02D)16
OPCODE n i x b p e Address
0011 11 1 1 0 0 1 0 (FEC)16
Base-Relative Addressing Modes
Base-relative
¾ Base register is under the control of the programmer
¾ 12 LDB #LENGTH
¾ 13 BASE LENGTH
¾ 160 104E STCH BUFFER, X 57C003
Displacement= BUFFER - B = 0036 - 0033 = 3
¾ NOBASE is used to inform the assembler that the contents of the base register no longer be relied upon for addressing
OPCODE n i x b p e Address
0101 01 1 1 1 1 0 0 (003)16
Immediate Address Translation
Immediate addressing
¾ 55 0020 LDA #3 010003
OPCODE n i x b p e Address
0000 00 0 1 0 0 0 0 (003)
¾ 133 103C +LDT #4096 75101000
0000 00 0 1 0 0 0 0 (003)16
OPCODE n i x b p e Address
0111 01 0 1 0 0 0 1 (01000)16
Immediate Address Translation
Immediate addressing
¾ 12 0003 LDB #LENGTH 69202D
¾ 12 0003 LDB #LENGTH 690033
OPCODE n i x b p e Address
0110 10 0 1 0 0 1 0 (02D)16
¾ 12 0003 LDB #LENGTH 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
OPCODE n i x b p e Address
0110 10 0 1 0 0 0 0 (033)16
Indirect Address Translation
Indirect addressing
¾ Target addressing is computed as usual (PC-relative or BASE- relative)
¾ Only the n bit is set to 1
¾ 70 002A J @RETADR 3E2003
TA=RETADR=0030
TA=(PC)+disp=002D+0003
OPCODE n i x b p e Address
0011 11 1 0 0 0 1 0 (003)16
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
Program Relocation
Examples of Program Relocation
Absolute program, starting address 1000
5 2000 1000 COPY START 1000
10 2000 1000 FIRST STL RETADR 141033 142033
15 2003 1003 CLOOP JSUB RDREC 482039 483039
20 2006 1006 LDA LENGTH 001036 002036
25 2009 1009 COMP ZERO 281030 282030
30 200C 100C JEQ ENDFIL 301015 302015
35 200F 100F JSUB WREC 482061 483061
40 2012 1012 J CLOOP 3C1003 3C2003
45 2015 1015 ENDFIL LDA EOF 00102A 00202A
50 2018 1018 STA BUFFER 0C1039 0C2039
55 201B 101B LDA THREE 00102D 00202D
60 201E 101E STA LENGTH 0C1036 0C2036
65 2021 1021 JSUB WREC 482061 483061
70 2024 1024 LDL RETADR 081033 082033
75 2027 1027 RSUB 4C0000 4C0000
80 202A 102A EOF BYTE C'EOF' 454E46 454E46
85 202D 102D THREE WORD 3 000003 000003
90 2030 1030 ZERO WORD 0 000000 000000
95 2033 1033 RETADR RESW 1
100 2036 1036 LENGTH RESW 1
105 2039 1039 BUFFER RESB 4096
Problems of Program Relocation
Except for absolute address, the rest of the instructions need not be modified
¾ not a memory address (immediate addressing)
¾ PC-relative, Base-relative
Th l t f th th t i difi ti
The only parts of the program that require modification
at load time are those that specify direct addresses
Examples of Program Relocation
5 1000 0000 COPY START 0
10 1000 0000 FIRST STL RETADR 17202D 17202D
12 1003 0003 LDB #LENGTH 69202D 69202D
13 BASE LENGTH
15 1006 0006 CLOOP +JSUB RDREC 4B101036 4B102036
20 100A 000A LDA LENGTH 032026 032026
25 100D 000D COMP #0 290000 290000
== Æ 1000
30 1010 0010 JEQ ENDFIL 332007 332007
35 1013 0013 +JSUB WRREC 4B10105D 4B10205D
40 1017 0017 J CLOOP 3F2FEC 3F2FEC
45 101A 001A ENDFIL LDA EOF 032010 032010
50 101D 001D STA BUFFER 0F2016 0F2016
55 1020 0020 LDA #3 010003 010003
60 1023 0023 STA LENGTH 0F200D 0F200D
65 1026 0026 +JSUB WRREC 4B10105D 4B10205D
70 102A 002A J @RETADR 3E2003 3E2003
80 102D 002D EOF BYTE C'EOF' 454F46 454F46
95 1030 0030 RETADR RESW 1
100 1036 0036 BUFFER RESB 4096
How to Make Program Relocation Easier
Use program-counter (PC) relative addresses
¾ Did you notice that we didn’t modify the addresses for JEQ, JLT and J instructions?
¾ We didn’t modify the addresses for RETADR, LENGTH, and BUFFER
BUFFER.
Virtual memory
¾ Every program pretends that it has all of memory. Therefore, Text segment always starts at a fixed address; Stack segment always resides a some huge high address.
Relocatable Program
Modification record
¾ Col 1 M
¾ Col 2-7 Starting location of the address field to be
modified, relative to the beginning of the program
Col 8 9 l th f th dd fi ld t b difi d i h lf
¾ Col 8-9 length of the address field to be modified, in half- bytes
Object Code with Modification Record
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
Program Blocks
Refer to segments of code that are rearranged within a single object program unit
USE [blockname]
At the beginning, statements are assumed to be part f th d (d f lt) bl k
of the unnamed (default) block
If no USE statements are included, the entire program belongs to this single block
Each program block may actually contain several
separate segments of the source program
Program Blocks - Implementation
Pass 1
¾ Each program block has a separate location counter
¾ Each label is assigned an address that is relative to the start of the block that contains it
¾ At the end of Pass 1, the latest value of the location ,
counter for each block indicates the length of that block
¾ The assembler can then assign to each block a starting address in the object program
Pass 2
¾ 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
Program Blocks - Implementation
Each source line is given a relative address assigned and a block number
For absolute symbol, there is no block number
¾ line 107
Example
¾ 20 0006 0 LDA LENGTH 032060
¾ LENGTH = (Block 1) + 0003 = 0066 + 0003 = 0069
¾ LOCCTR = (Block 0) + 0009 = 0009
Object Code
Loading Program Blocks
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
Control Section
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
I t ti i t l ti d t f t
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
¾ EXTDEF name [, name]
¾ EXTDEF names symbols that are defined in this control section and may be used by other sections
External reference
¾ EXTREF name [,name]
¾ EXTREF name [,name]
¾ EXTREF names symbols that are used in this control section and are defined elsewhere
Example
¾ 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
¾ Col. 1 D
¾ Col. 2-7 Name of external symbol defined in this control section
¾ Col. 8-13 Relative address within this control section (hexadeccimal)
¾ Col.14-73 Repeat information in Col. 2-13 for other external symbols
Refer record
¾ Col. 1 D
¾ Col. 2-7 Name of external symbol referred to in this control section
¾ Col. 8-73 Name of other external reference symbols
Modification Record
Modification record
¾ Col. 1 M
¾ Col. 2-7 Starting address of the field to be modified (hexiadecimal)
¾ Col. 8-9 Length of the field to be modified, in half-bytes (hexadeccimal)
¾ Col.11-16 External symbol whose value is to be added to or subtracted from the indicated field
N l i i i ll l b l i i i
¾ Note: control section name is automatically an external symbol, i.e. it is available for use in Modification records.
Object Code
Functions of a Basic Assembler
Object File
Address Translation
Program Relocation
Design & Implementation of Assembler
Program Block
Control Section & Program Linking
Other Issues
¾ One-pass Assembler
¾ Multi-pass Assembler
One-Pass Assemblers
Main problem
¾ Forward references
Data items & labels on instructions
Solution
¾ Require all such areas be defined before they are referencedequ e suc e s be de ed be o e ey e e e e ced
¾ Labels on instructions: no good solution
Two types of one-pass assemblers
¾ Load-and-go
Produces object code directly in memory for immediate execution
¾ The other
Produces usual kind of object code for later execution
Load-and-go Assembler
Characteristics
¾ Useful for program development and testing
¾ Avoids the overhead of writing the object program out and reading it back
¾ Both one-pass and two-pass assemblers can be designed as p p g load-and-go.
¾ However one-pass also avoids the over head of an additional pass over the source program
¾ For a load-and-go assembler, the actual address must be known at assembly time, we can use an absolute program
Load-and-go Assembler
Forward references handling
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 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
At the end of the program
¾ Any SYMTAB entries that are still marked with * indicate undefined symbols
¾ Search SYMTAB for the symbol named in the END statement and jump to this location to begin executionj p g
The actual starting address must be specified at
assembly time
After Scanning Line 40
After Scanning Line 160
Object Code
Multi-Pass Assemblers
Restriction on EQU
¾ No forward reference, since symbols’ value can’t be defined during the first pass
Example
Use link list to keep track of hose al e depend on an
¾ Use link list to keep track of whose value depend on an undefined symbol
