2. Chap 2
Role of Assembler
Role of Assembler
Source
Program
Assembler
Object
Code
Loader
Executable
Code
Linker
3. Chap 2
Chapter 2 -- Outline
Chapter 2 -- Outline
Basic Assembler Functions
Machine-dependent Assembler Features
Machine-independent Assembler Features
Assembler Design Options
4. Chap 2
Introduction to Assemblers
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
5. Chap 2
Example Program (Fig. 2.1)
Example Program (Fig. 2.1)
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
6. Chap 2
Example Program (Fig. 2.1)
Example Program (Fig. 2.1)
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
7. Chap 2
Assembler Directives
Assembler Directives
Pseudo-Instructions
Not translated into machine instructions
Providing information to the assembler
Basic assembler directives
START
END
BYTE
WORD
RESB
RESW
8. Chap 2
Object Program
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)
9. Chap 2
Fig. 2.3
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
10. Chap 2
Figure 2.1 (Pseudo code)
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
}
11. Chap 2
An Example (Figure 2.1,
An Example (Figure 2.1, Cont
Cont.)
.)
Subroutine RDREC {
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
}
EOR:
character x‘00’
12. Chap 2
An Example (Figure 2.1,
An Example (Figure 2.1, Cont
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
}
13. Chap 2
Assembler’s functions
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
14. Chap 2
Example of Instruction Assemble
Example of Instruction Assemble
Forward reference
STCH BUFFER,X
(54)16 1 (001)2 (039)16
8 1 15
opcode x address
m
549039
15. Chap 2
Difficulties: 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
16. Chap 2
Two Pass Assembler
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
17. Chap 2
Two Pass Assembler
Two Pass Assembler
Read from input line
LABEL, OPCODE, OPERAND
Pass 1 Pass 2
Intermediate
file
Object
codes
Source
program
OPTAB SYMTAB SYMTAB
18. Chap 2
Data Structures
Data Structures
Operation Code Table (OPTAB)
Symbol Table (SYMTAB)
Location Counter(LOCCTR)
19. Chap 2
OPTAB (operation code table)
OPTAB (operation code table)
Content
menmonic, machine code (instruction format,
length) etc.
Characteristic
static table
Implementation
array or hash table, easy for search
20. Chap 2
SYMTAB (symbol table)
SYMTAB (symbol table)
Content
label name, value, flag, (type, length) etc.
Characteristic
dynamic table (insert, delete, search)
Implementation
hash table, non-random keys, hashing function
COPY 1000
FIRST 1000
CLOOP 1003
ENDFIL 1015
EOF 1024
THREE 102D
ZERO 1030
RETADR 1033
LENGTH 1036
BUFFER 1039
RDREC 2039
21. Chap 2
Homework #3
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
22. Chap 2
Assembler Design
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
24. Chap 2
Instruction Format and 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)
Example program
25. Chap 2
Translation
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
26. Chap 2
PC-Relative Addressing Modes
PC-Relative Addressing Modes
PC-relative
10 0000 FIRST STL RETADR 17202D
(14)16 1 1 0 0 1 0 (02D) 16
displacement= RETADR - PC = 30-3 = 2D
40 0017 J CLOOP 3F2FEC
(3C)16 1 1 0 0 1 0 (FEC) 16
displacement= CLOOP-PC= 6 - 1A= -14= FEC
op(6) n I x b p e disp(12)
op(6) n I x b p e disp(12)
27. Chap 2
Base-Relative Addressing Modes
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 )16 1 1 1 1 0 0 ( 003 ) 16
(54) 1 1 1 0 1 0 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
op(6) n I x b p e disp(12)
28. Chap 2
Immediate Address Translation
Immediate Address Translation
Immediate addressing
55 0020 LDA #3 010003
( 00 )16 0 1 0 0 0 0 ( 003 ) 16
133 103C +LDT #4096 75101000
( 74 )16 0 1 0 0 0 1 ( 01000 ) 16
op(6) n I x b p e disp(12)
op(6) n I x b p e disp(20)
29. Chap 2
Immediate Address Translation
Immediate Address Translation (Cont.)
(Cont.)
Immediate addressing
12 0003 LDB #LENGTH 69202D
( 68)16 0 1 0 0 1 0 ( 02D ) 16
( 68)16 0 1 0 0 0 0 ( 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
op(6) n I x b p e disp(12)
30. Chap 2
Indirect Address Translation
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
( 3C )16 1 0 0 0 1 0 ( 003 ) 16
TA=RETADR=0030
TA=(PC)+disp=002D+0003
op(6) n I x b p e disp(12)
31. Chap 2
Program Relocation
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
33. Chap 2
Relocatable Program
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
36. Chap 2
Literals
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
37. Chap 2
Literals vs. Immediate Operands
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
38. Chap 2
Literal - Implementation (1/3)
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
39. Chap 2
Literal - Implementation (2/3)
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
40. Chap 2
Literal - Implementation (3/3)
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
41. Chap 2
Symbol-Defining Statements
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
42. Chap 2
Symbol-Defining Statements
Symbol-Defining Statements
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
+LDT
#4096
43. Chap 2
ORG (origin)
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
SYMBOL VALUE FLAGS
STAB
(100 entries)
. . .
. . .
. . .
44. Chap 2
ORG Example
ORG Example
Using EQU statements
STAB RESB 1100
SYMBOL EQU STAB
VALUEEQU STAB+6
FLAG EQU STAB+9
Using ORG statements
STAB RESB 1100
ORG STAB
SYMBOL RESB 6
VALUERESW 1
FLAGSRESB 2
ORG STAB+1100
45. Chap 2
Expressions
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
46. Chap 2
SYMTAB
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
47. Chap 2
Example 2.9
Example 2.9
SYMTAB LITTAB
Name Value
COPY 0
FIRST 0
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
C'EOF' 454F46 3 002D
X'05' 05 1 1076
48. Chap 2
Program Blocks
Program Blocks
Program blocks
refer to segments of code that are rearranged
within a single object program unit
USE [blockname]
Default block
Example: Figure 2.11
Each program block may actually contain
several separate segments of the source
program
49. Chap 2
Program Blocks - Implementation
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
50. Chap 2
Figure 2.12
Figure 2.12
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
51. Chap 2
Program Readability
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
see Fig. 2.13, Fig. 2.14
53. Chap 2
Control Sections
Control Sections and Program Linking
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
Fig. 2.15, 2.16
54. Chap 2
External Definition and References
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
55. Chap 2
Implementation
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
Col. 8-73 Name of other external reference symbols
56. Chap 2
Modification Record
Modification Record
Modification record
Col. 1 M
Col. 2-7Starting address of the field to be modified (hexiadecimal)
Col. 8-9Length 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
57. Chap 2
External References in Expression
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.
59. Chap 2
Two-Pass Assembler with Overlay
Two-Pass Assembler with Overlay
Structure
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
60. Chap 2
One-Pass Assemblers
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
61. Chap 2
One-Pass Assemblers
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
62. Chap 2
Load-and-go Assembler
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
63. Chap 2
Forward Reference in One-pass Assembler
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
64. Chap 2
Load-and-go Assembler (Cont.)
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
Figure 2.18, 2.19
65. Chap 2
Producing Object Code
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: Figure 2.20
66. Chap 2
Multi-Pass Assemblers
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
Figure 2.21
![Chap 2
An Example (Figure 2.1,
An Example (Figure 2.1, Cont
Cont.)
.)
Subroutine RDREC {
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
}
EOR:
character x‘00’](https://image.slidesharecdn.com/mod5-250605040906-8f9798d8/85/Mod-5-1-Assembler-Summaryyyyyyyyyyyyyyy-ppt-11-320.jpg)
![Chap 2
An Example (Figure 2.1,
An Example (Figure 2.1, Cont
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
}](https://image.slidesharecdn.com/mod5-250605040906-8f9798d8/85/Mod-5-1-Assembler-Summaryyyyyyyyyyyyyyy-ppt-12-320.jpg)
![Chap 2
Program Blocks
Program Blocks
Program blocks
refer to segments of code that are rearranged
within a single object program unit
USE [blockname]
Default block
Example: Figure 2.11
Each program block may actually contain
several separate segments of the source
program](https://image.slidesharecdn.com/mod5-250605040906-8f9798d8/85/Mod-5-1-Assembler-Summaryyyyyyyyyyyyyyy-ppt-48-320.jpg)
![Chap 2
External Definition and References
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](https://image.slidesharecdn.com/mod5-250605040906-8f9798d8/85/Mod-5-1-Assembler-Summaryyyyyyyyyyyyyyy-ppt-54-320.jpg)
