unit ii assemblers - notes · pdf fileintroduction to assemblers ... sic instruction format...
TRANSCRIPT
1
UNIT II ASSEMBLERS
www.noyesengine.com
www.technoscriptz.com
2
1. BASIC ASSEMBLER FUNCTIONS
2. A SIMPLE SIC ASSEMBLER
3. ASSEMBLER ALGORITHM AND DATA STRUCTURES
4. MACHINE DEPENDENT ASSEMBLER FEATURES
5. INSTRUCTION FORMATS AND ADDRESSING MODES
6. PROGRAM RELOCATION
7. MACHINE INDEPENDENT ASSEMBLER FEATURES
8. LITERALS
9. SYMBOL-DEFINING STATEMENTS
3
10. EXPRESSIONS
11. ONE PASS ASSEMBLERS
12. MULTI PASS ASSEMBLERS
13. IMPLEMENTATION EXAMPLE
14. MASM ASSEMBLER
Assemblers
Basic Assembler Functions
Machine-dependent Assembler Features
Machine-independent Assembler Features
Assembler Design Options
Role of Assembler
4
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
Source
Program
Assembler Object
Code
Loader
Executable
Code
Linker
5
Example Program
Object Code
6
7
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
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
8
Providing information to the assembler
Basic assembler directives START
END
BYTE
WORD
RESB
RESW
Functions of a Basic Assembler Mnemonic code (or instruction name) opcode.
Symbolic operands (e.g., variable names) addresses.
Choose the proper instruction format and addressing mode.
Constants Numbers.
Output to object files and listing files.
SIC Instruction Set Load/Store: LDA/STA, LDX/STX…etc.
Arithmetic: ADD, SUB, MUL, DIV
Compare: COMP
Jump: J
Conditional Jump: JLT, JEQ, JGT
See Appendix A for the complete list.
SIC Instruction Format
Opcode: 8 bits Address: one bit flag (x) and 15 bits of address
Examples
Mnemonic code (or instruction name) Opcode. Variable names, Labels, Subroutines, Constants Address
Examples:
OPCODE X Address
8 1 15
9
STL RETADR 14 10 33
STCH BUFFER,X 54 90 39
Converting Symbols to Numbers
Isn’t it straightforward? Isn’t it simply the sequential processing of the source program, one line at
a time? Not so, if we have forward references.
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
Read from input line LABEL, OPCODE, OPERAND
COPY START 1000
…
LDA LEN
…
…
LEN RESW 1
0001 0100 0 001 0000 0011 0011
0101 0100 1 001 0000 0011 1001
10
Two Pass Assembler – Pass 1
• Assign addresses to all statements in the program
• Save the values (addresses) assigned to all labels for use in Pass 2
• Perform some processing of assembler directives. (This includes processing that affects address
assignment, such as determining the length of data areas defined by BYTE, RESW, etc.
PASS-I Algorithm
begin
read first input line
if OPCODE = 'START' then
begin
save #[OPERAND] as starting address
initialized LOCCTR to starting address
write line to intermediate file
read next input line
end {if START}
else
initialized LOCCTR to 0
while OPCODE != 'END' do
begin
if this is not a comment line then
begin
if there is a symbol in the LABEL field then
begin
search SYMTAB for LABEL
if found then
set error flag (duplicate symbol)
else
insert (LABEL, LOCCTR) into SYMTAB
Pass 1 Pass 2
Intermediat
e
file
Object
codes
Source
progra
m
OPTAB SYMTAB SYMTAB
11
end {if symbol}
search OPTAB for OPCODE
if found then
add 3 {instruction lengh} to LOCCTR
else if OPCODE = 'WORD' then
add 3 to LOCCTR
else if OPCODE = 'RESW' then
add 3 * #[OPERAND] to LOCCTR
else if OPCODE = 'RESB' then
add #[OPERAND] to LOCCTR
else if OPCODE = 'BYTE' then
begin
find length of constant in bytes
add length to LOCCTR
end {if BYTE}
else
set error flag (invalid operation code)
end {if not a comment}
write line to intermediate file
read next input line
end {while not END}
write last line to intermediate file
save (LOCCTR - starting address) as program length
end
Two Pass Assembler – Pass 2
• Assemble instructions (translating operation codes and looking up addresses).
• Generate data values defined by BYTE, WORD, etc.
• Perform processing of assembler directives not done during Pass 1
• Write the object program and the assembly listing
PASS-2 Algorithm
begin
read first input file {from intermediate file}
if OPCODE = 'START' then
begin
write listing line
read next input line
end {if START}
write header record to object program
initialized first Text record
while OPCODE != 'END' do
begin
if this is not a comment line then
12
begin
search OPTAB for OPCODE
if found then
begin
if there is a symbol in OPERAND field then
begin
search SYMTAB for OPERAND
if found then
store symbol value as operand address
else
begin
store 0 as operand address
set error flag (undefined symbol)
end
end {if symbol}
else
store 0 as operand address
assemble the object code instruction
end {if opcode found}
else if OPCODE = 'BYTE' or 'WORD' then
convert constant to object code
if object code not fit into the current Text record then
begin
write Text record to object program
initialized new Text record
end
add object code to Text record
end {if not comment}
write listing line
read next input line
end {while not END}
write last Text record to object program
write End record to object program
write last listing line
end
Data Structures
Operation Code Table (OPTAB) Symbol Table (SYMTAB) Location Counter(LOCCTR)
OPTAB (operation code table)
• Operation Code Table (OPTAB)
– Used to look up mnemonic operation codes and translate them into machine language
equivalents
– Contains the mnemonic operation code and its machine language equivalent
– In more complex assemblers, contains information like instruction format and length
13
SYMTAB (symbol table)
Used to store values (addresses) assigned to labels
Includes the name and value for each label
Flags to indicate error conditions, e.g. duplicate definition of labels
May contain other info like type or length about the data area or instruction labeled
LOCCTR(Location counter) – Used to help in the assignment of addresses – Initialized to the beginning address specified in the START
statement – After each source statement is processed, the length of the
assembled instruction or data area to be generated is added – Gives the address of a label
COPY 1000
FIRST 1000
CLOOP 1003
ENDFIL 1015
EOF 1024
THREE 102D
ZERO 1030
RETADR 1033
LENGTH 1036
BUFFER 1039
RDREC 2039
14
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.2~7Starting 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.2~7Address of first executable instruction (hex)
(END program_name)
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
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
15
Machine-dependent Assembler Features
IInnssttrruuccttiioonn ffoorrmmaattss aanndd aaddddrreessssiinngg mmooddeess
PPrrooggrraamm rreellooccaattiioonn
For Eg STL RETADR 17 20 2D
The mode bit p=1, meaning PC relative addressing mode.
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
OPCODE e Address
6
bits
12 bits
n i x b p
0001 01 0 0000 0010 1101 1 1 0 0 1
1
7
2D 2
0
16
PC-Relative Addressing Modes
PC-relative 10 0000 FIRST STL RETADR 17202D
displacement= RETADR - PC = 30-3 = 2D
40 0017 J CLOOP 3F2FEC
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
OPCODE e Address n i x b p
0001 01 0 (02D)16 1 1 0 0 1
OPCODE e Address n i x b p
0011 11 0 (FEC)16 1 1 0 0 1
17
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
133 103C +LDT #4096 75101000
Immediate Address Translation (Cont.)
Immediate addressing 12 0003 LDB #LENGTH 69202D
OPCODE e Address n i x b p
0101 01 0 (003)16 1 1 1 1 0
OPCODE e Address n i x b p
0000 00 0 (003)16 0 1 0 0 0
OPCODE e Address n i x b p
0111 01 1 (01000)16 0 1 0 0 0
18
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
TA=RETADR=0030
TA=(PC)+disp=002D+0003
Program Relocation
OPCODE e Address n i x b p
0110 10 0 (02D)16 0 1 0 0 1
OPCODE e Address n i x b p
0011 11 0 (003)16 1 0 0 0 1
19
Example 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 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
Relocatable Program
Modification record Col 1 M
Col 2-7 Starting location of the address field to be
20
modified, relative to the beginning of the program
Col 8-9 length of the address field to be modified, in half-
bytes
Object Code with M-Records
Modification records are added to the object files. (See pp.64-65 and Figure 2.8.)
Example:
HCOPY 001000 001077
T000000 1D 17202D…4B101036…
T00001D ……
…
M000007 05 Modification Record
……
E000000
Object Code
Machine-Independent Assembler Features LLiitteerraallss
SSyymmbbooll DDeeffiinniinngg SSttaatteemmeenntt
EExxpprreessssiioonnss
PPrrooggrraamm BBlloocckkss
CCoonnttrrooll SSeeccttiioonnss aanndd PPrrooggrraamm LLiinnkkiinngg
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
21
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 ENDFIL LDA =C’EOF’ 032010
Compare (Fig. 2.6) e.g. 45 001A ENDFIL LDA EOF 032010
80 002D EOF BYTE C’EOF’ 454F46
Literal - Implementation
Literal pools Normally literals are placed into a pool at the end of the program
see (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
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
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
C'EOF' 454F46 3 002D
X'05' 05 1 1076
22
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
Example 1 MAXLEN EQU 4096
+LDT #MAXLEN
Example 2 (Many general purpose registers) BASE EQU R1
COUNT EQU R2
INDEX EQU R3
Example 3 MAXLEN EQU BUFEND-BUFFER
Counter Example
BETA EQU ALPHA
ALPHA RESW 1
BETA cannot be assigned a value when it is encountered during Pass 1 of the assembly
(because ALPHA does not yet have a value).
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
VALUE: 1word
FLAGS: 2bytes LDA VALUE, X
+LDT #4096
23
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
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
SYMBOL VALUE FLAGS
STAB
(100 entries)
. . .
. . .
. . .
Symbol Type Value
RETADR R 30
BUFFER R 36
BUFEND R 1036
MAXLEN A 1000
24
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
Each source line is given a relative address assigned and a block number
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
Block name Block number Address Length
(default) 0 0000 0066
CDATA 1 0066 000B
CBLKS 2 0071 1000
25
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
26
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 D
Col. 2-7 Name of external symbol referred to in this control section
27
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 Figure 2.17
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.
Assembler Design Options OOnnee--ppaassss aasssseemmbblleerrss
MMuullttii--ppaassss aasssseemmbblleerrss
TTwwoo--ppaassss aasssseemmbblleerr wwiitthh oovveerrllaayy ssttrruuccttuurree
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
28
data items
labels on instructions
Solution data items: require all such areas be defined before they are referenced
labels on instructions: no good solution
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
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
29
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
30
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
31
32
33
ASSEMBLERS
1. Introduction
There are two main classes of programming languages: high level (e.g., C, Pascal) and low
level. Assembly Language is a low level programming language. Programmers code symbolic
instructions, each of which generates machine instructions.
An assembler is a program that accepts as input an assembly language program (source) and
produces its machine language equivalent (object code) along with the information for the loader.
Assembly language program
Assembler Linker EXE
Figure 1. Executable program generation from an assembly source code
Advantages of coding in assembly language are:
Provides more control over handling particular hardware components
May generate smaller, more compact executable modules
Often results in faster execution
Disadvantages:
Not portable
More complex
Requires understanding of hardware details (interfaces)
Assembler:
An assembler does the following:
1. Generate machine instructions
- evaluate the mnemonics to produce their machine code
34
- evaluate the symbols, literals, addresses to produce their equivalent machine addresses
- convert the data constants into their machine representations
2. Process pseudo operations
2. Two Pass Assembler
A two-pass assembler performs two sequential scans over the source code:
Pass 1: symbols and literals are defined
Pass 2: object program is generated
Parsing: moving in program lines to pull out op-codes and operands
Data Structures:
- Location counter (LC): points to the next location where the code will be placed
- Op-code translation table: contains symbolic instructions, their lengths and their op-codes (or
subroutine to use for translation)
- Symbol table (ST): contains labels and their values
- String storage buffer (SSB): contains ASCII characters for the strings
- Configuration table: contains pointer to the string in SSB and offset where its value will be inserted
in the object code
assembly machine
language Pass1 Pass 2 language
program Symbol table program
Configuration table
String storage buffer
Partially configured object file
Figure 2. A simple two pass assembler.
35
Example 1: Decrement number 5 by 1 until it is equal to zero.
assembly language memory object code
program address in memory
-----------------------
START 0100H
LDA #5 0100 01
0101 00
0102 05
LOOP: SUB #1 0103 1D
0104 00
0105 01
COMP #0 0106 29
0107 00
0108 00
JGT LOOP 0109 34
010A 01 placed in Pass 1
010B 03
RSUB 010C 4C
010D 00
010E 00
END
Op-code Table
Mnemonic Addressing
mode
Opcode
LDA immediate 01
SUB immediate 1D
COMP immediate 29
LDX immediate 05
ADD indexed 18
TIX direct 2C
JLT direct 38
JGT direct 34
RSUB implied 4C
Symbol Table
Symbo
l
Value
LOOP 0103
36
Example 2: Sum 6 elements of a list which starts at location 200.
assembly language memory object code
program address in memory
-----------------------
START 0100H
LDA #0 0100 01
0101 00
0102 00
LDX #0 0103 05
0104 00
0105 00
LOOP: ADD LIST, X 0106 18
0107 01 placed in Pass 2
0108 12
TIX COUNT 0109 2C
010A 01 placed in Pass 2
010B 15
JLT LOOP 010C 38
010D 01 placed in Pass 1
010E 06
RSUB 010F 4C
0110 00
0111 00
LIST: WORD 200 0112 00
0113 02
0114 00
COUNT: WORD 6 0115 00
0116 00
0117 06
END
Symbol Table
Symbo
l
Addres
s
LOOP 0106
37
LIST 0112
COUN
T
0115
Configuration Table
Offset SSB pointer
for the
symbol
0007 DC00
000A DC05
SSB
DC00 4C
H
DC01 49H ASCII for L,I,S,T
DC02 53H
DC03 54H
DC04 5E
H
ASCII for separation
character
DC05
Pass1
All symbols are identified and put in ST
All op-codes are translated
Missing symbol values are marked
LC = origin
Read next statement
Parse the statement
Y
Comment
N
“END” Y Pass 2
38
N
N pseudo-op Y what
kind?
N
Label EQU WORD/ RESW/RESB
BYTE
Y
N Label N Label
Enter label in ST Enter label in ST
Y Y
Enter label in ST Enter label in ST
Call translator
Place constant in
machine code
Advance LC by the
number of bytes specified
Advance LC in the pseudo-op
Figure 3. First pass of a two-pass assembler.
39
Translator Routine
Find opcode and the number of bytes
in Op-code Table
Write opcode in machine code
Write the data or address that is known
at this time in machine code
more
information Y
will be needed Set up an entry in
in Pass 2 ? Configuration Table
N
Advance LC by the number of bytes
in op-code table
return
Figure 4. Flowchart of a translator routine
Pass 2
- Fills addresses and data that was unknown during Pass 1.
Pass 1
More lines in N
Configuration Done
Table
Y
Get the next line
Retrieve the name of the symbol from SSB
Get the value of the symbol from ST
40
Compute the location in memory
where this value will be placed
(starting address + offset)
Place the symbol value at this location
Figure 5. Second pass of a two-pass assembler.
Relocatable Code
Producing an object code, which can be placed to any specific area in memory.
Direct Address Table (DAT): contains offset locations of all direct addresses in the program (e.g., 8080
instructions that specify direct addresses are LDA, STA, all conditional jumps...). To relocate the
program, the loader adds the loading point to all these locations.
assembly language program Assembler machine language program
and DAT
Figure 6. Assembler output for a relocatable code.
Example 3: Following relocatable object code and DAT are generated for Example 1.
assembly language memory object code
program address in memory
-----------------------
START
LDA #0 0000 01
0001 00
0002 00
LDX #0 0003 05
0004 00
0005 00
LOOP: ADD LIST, X 0006 18
0007 00
0008 12
TIX COUNT 0009 2C
000A 00
000B 15
JLT LOOP 000C 38
000D 00
000E 06
RSUB 000F 4C
0010 00
0011 00
LIST: WORD 200 0012 00
0013 02
41
0014 00
COUNT: WORD 6 0015 00
0016 00
0017 06
END
DAT
0007
000A
000D
Forward and backward references in the machine code are generated relative to address 0000. To
relocate the code, the loader adds the new load-point to the references in the machine code which are
pointed by the DAT.
One-Pass Assemblers
Two methods can be used:
- Eliminating forward references
Either all labels used in forward references are defined in the source program before they are
referenced, or forward references to data items are prohibited.
- Generating the object code in memory
No object program is written out and no loader is needed. The program needs to be re-assembled
every time.
Multi-Pass Assemblers
Make as many passes as needed to process the definitions of symbols.
Example 3:
A EQU B
B EQU C
C DS 1
3 passes are required to find the address for A.
42
Such references can also be solved in two passes: entering symbol definitions that
involve forward references in the symbol table. Symbol table also indicates which
symbols are dependent on the values of others.
Example 4:
A EQU B
B EQU D
C EQU D
D DS 1
At the end of Pass1:
Symbol Table
A &1 B 0
B &1 D A 0
C &1 D 0
D 200 B C 0
After evaluating dependencies:
Symbol Table
A 200 0
B 200 0
C 200 0
D 200 0