Tuesday, 20 September 2016

Assembler
















Role of Assembler
Source
Assembler
Object
Linker
Program
Code







Executable

Code





Loader







2
Chapter 2 -- Outline


n       Basic Assembler Functions

n       Machine-dependent Assembler Features

n       Machine-independent Assembler Features

n       Assembler Design Options
























3
Introduction to Assemblers


n       Fundamental functions

u    translating mnemonic operation codes to their machine language equivalents

u    assigning machine addresses to symbolic labels



n       Machine dependency

u    different machine instruction formats and codes











4
Example Program (Fig. 2.1)





n       Purpose

u    reads records from input device (code F1)

u    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



















5
Example Program (Fig. 2.1)


n       Data transfer (RD, WD)

u    a buffer is used to store record

u    buffering is necessary for different I/O rates

u    the end of each record is marked with a null character (0016)

u    the end of the file is indicated by a zero-length record

n       Subroutines (JSUB, RSUB)

u    RDREC, WRREC

u    save link register first before nested jump




6
Assembler Directives


n       Pseudo-Instructions

u    Not translated into machine instructions

u    Providing information to the assembler

n       Basic assembler directives

u    START

u    END

u    BYTE

u    WORD

u    RESB

u    RESW



7
Assembler’s functions



n           Convert mnemonic operation codes to their machine language equivalents

n       Convert symbolic operands to their equivalent machine addresses

n       Build the machine instructions in the proper format

n       Convert the data constants to internal machine representations

n       Write the object program and the assembly listing


8
Example of Instruction Assemble
STCH
BUFFER,X
549039











8
1

15













opcode

x

address






















m


(54)16
1
(001)2
(039)16


n  Forward reference














9
Difficulties: Forward Reference


n   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




















10
Two Pass Assembler


n   Pass 1

u Assign addresses to all statements in the program

u Save the values assigned to all labels for use in Pass 2

u Perform some processing of assembler directives

n   Pass 2

u Assemble instructions

u Generate data values defined by BYTE, WORD

u Perform processing of assembler directives not done in Pass 1

u Write the object program and the assembly listing








11
Two Pass Assembler


n   Read from input line

u LABEL, OPCODE, OPERAND





Source program
Pass 1
Intermediate
Pass 2
Object

file



codes
OPTAB

SYMTAB
SYMTAB












12
Data Structures





n   Operation Code Table (OPTAB)

n   Symbol Table (SYMTAB)

n   Location Counter(LOCCTR)


























13
OPTAB (operation code table)




n   Content

u menmonic, machine code (instruction format, length) etc.

n   Characteristic

u static table

n   Implementation

u array or hash table, easy for search












14
SYMTAB (symbol table)



n  Content                                                                                                             COPY
ulabel name, value, flag, (type,FIRST

CLOOP
n
Characteristic
ENDFIL

udynamic table (insert, delete,THREEEOF
n
Implementation
ZERO

uhash table, non-random key LENGTHRETADR
BUFFER

RDREC



1000

etc.10001003


1015
1024

102D

1030
1033

g function1036

1039

2039












15
Object Program
n
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)
n
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
n
End


Col.1
E

Col.2~7
Address of first executable instruction (hex)


(END program_name)


16
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





















17
Homework #1
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








18
Assembler Design


n   Machine Dependent Assembler Features

u instruction formats and addressing modes

u program relocation

n   Machine Independent Assembler Features

u literals

u symbol-defining statements

u expressions

u program blocks

u control sections and program linking











19
Machine-dependent
Assembler Features


Sec. 2-2

n Instruction formats and addressing modes

n Program relocation


























20
Instruction Format and Addressing Mode




n  SIC/XE
u PC-relative or Base-relative addressing:
op m
u Indirect addressing:
op @m
u Immediate addressing:
op #c
u Extended format:
+op m
u Index addressing:
op m,x
u register-to-register instructions

u larger memory -> multi-programming (program allocation)

n   Example program

u Figure 2.5






21
Translation


n   Register translation

u register name (A, X, L, B, S, T, F, PC, SW) and their values (0,1, 2, 3, 4, 5, 6, 8, 9)

u preloaded in SYMTAB

n   Address translation

u Most register-memory instructions use program counter relative or base relative addressing

u Format 3: 12-bit address field

F base-relative: 0~4095
pc-relative: -2048~2047
F

u Format 4: 20-bit address field






22
PC-Relative Addressing Modes





n  PC-relative

u 10
0000
FIRST STL
RETADR
17202D


















op(6)
n
I
x
b
p
e


disp(12)






(14)16
1 1 0 0 1 0

(02D) 16




F displacement= RETADR - PC = 30-3 = 2D
u 40
0017




J
CLOOP
3F2FEC
















op(6)

n
I
x
b
p
e

disp(12)






(3C)16
1 1 0 0 1 0
(FEC) 16




F displacement= CLOOP-PC= 6 - 1A= -14= FEC

23
Base-Relative Addressing Modes
n  Base-relative
u base register is under the control of the programmer

u 12




LDB
#LENGTH
u 13




BASE
LENGTH
u 160
104E



STCH
BUFFER, X   57C003















op(6)
n
I
x
b
p
e

disp(12)
















( 54 )16
1 1 1 1 0 0
( 003 ) 16




(54)
1 1 1 0 1 0   0036-1051= -101B










16


F displacement= BUFFER - B = 0036 - 0033 = 3

u NOBASE is used to inform the assembler that the contents of the base register no longer be relied upon for addressing

24
Immediate Address Translation





n  Immediate addressing
u 55
0020



LDA
#3
010003
















op(6)
n
I
x
b
p
e


disp(12)

( 00 )16
0 1 0 0 0 0
( 003
) 16


u 133
103C



+LDT
#4096
75101000

op(6)
n
I
x
b
p
e


disp(20)






( 74
)16
0 1 0 0 0 1
( 01000 ) 16






















25
Immediate Address Translation (Cont.)





n  Immediate addressing
u 12
0003





LDB   #LENGTH
69202D














op(6)

n
I
x
b
p
e

disp(12)


( 68)16


0 1 0 0 1 0
( 02D ) 16


( 68)16


0 1 0 0 0 0
( 033)16
690033
F the immediate operand is the symbol LENGTH

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

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

F if immediate mode is specified, the target address becomes the operand

26
Indirect Address Translation


n  Indirect addressing
utarget addressing is computed as usual (PC-

relative or BASE-relative)
uonly the n bit is set to 1
u 70
002A



J
@RETADR   3E2003












op(6)
n
I
x
b
p
e
disp(12)













( 3C )16
1 0 0 0 1 0
( 003 ) 16


F TA=RETADR=0030

F TA=(PC)+disp=002D+0003






27
Program Relocation

n  Example Fig. 2.1
uAbsolute program, starting address 1000
e.g. 55         101B                                           LDA              THREE                                     00102D

uRelocate the program to 2000
e.g. 55         101B                                           LDA              THREE                                     00202D

u Each Absolute address should be modified

n   Example Fig. 2.5:

u Except for absolute address, the rest of the instructions
need not be modified

F not a memory address (immediate addressing)

F PC-relative, Base-relative

u The only parts of the program that require modification at load time are those that specify direct addresses

28
Example











































29
Relocatable Program







n   Modification record

u Col 1  M

u Col 2-7 Starting location of the address field to be
modified, relative to the beginning of the program uCol 8-9 length of the address field to be modified, in half-

bytes














30
Object Code












































31
Machine-Independent Assembler
Features


Literals

Symbol Defining Statement
Expressions

Program Blocks
Control Sections and Program

Linking















32
Literals

n   Design idea

u Let programmers to be able to write the value of a constant operand as a part of the instruction that uses it.

u This avoids having to define the constant elsewhere in the program and make up a label for it.

n   Example
u e.g. 45
001A
ENDFILLDA
=C’EOF’
032010
u
93


LTORG


u
002D
*
=C’EOF’

454F46
u e.g. 215
1062
WLOOP
TD
=X’05’
E32011






33
Literals vs. Immediate Operands



n  Immediate Operands
u The operand value is assembled as part of the machine instruction
u e.g. 55  0020                                                                         LDA             #3                                                                                                                                                010003

n  Literals
u The assembler generates the specified value as a constant at some other memory location

u e.g. 45   001A   ENDFILLDA   =C’EOF’
032010
n  Compare (Fig. 2.6)

u e.g. 45
001A
ENDFIL
LDA
EOF  032010
u
80
002D
EOF
BYTE
C’EOF’454F46


34
Literal - Implementation (1/3)


n   Literal pools

u Normally literals are placed into a pool at the end of the program
F see Fig. 2.10 (END statement)
In some cases, it is desirable to place literals
u

into a pool at some other location in the object program

F assembler directive LTORG

F reason: keep the literal operand close to the instruction





35
Literal - Implementation (2/3)

n  Duplicate literals
ue .g . 215
1062 WLOOP
TD
=X’05’
ue.g. 230
106B
WD
=X’05’
u The assemblers should recognize duplicate literals and store only one copy of the specified data value
F Comparison of the defining expression

•     Same literal name with different value, e.g. LOCCTR=*

F 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


36
Literal - Implementation (3/3)


n        LITTAB

u literal name, the operand value and length, the address assigned to the operand
Pass 1
n

u build LITTAB with literal name, operand value and length, leaving the address unassigned

u when LTORG statement is encountered, assign an address to each literal not yet assigned an address
n   Pass 2

u search LITTAB for each literal operand encountered

u generate data values using BYTE or WORD statements

u generate modification record for literals that represent an address in the program

37
Symbol-Defining Statements


n   Labels on instructions or data areas

u the value of such a label is the address assigned to the statement

n   Defining symbols

u symbolEQU value

u value can be: ¬ constant, ­ other symbol, ® expression

u making the source program easier to understand

u no forward reference




38
Symbol-Defining Statements

n  Example 1
u MAXLEN
EQU
4096

u
+LDT
#MAXLEN
+LDT   #4096
n  Example 2

u BASE        EQU           R1
u COUNT EQU       R2

u INDEX  EQU         R3

n  Example 3

u MAXLEN                          EQU           BUFEND-BUFFER









39
ORG (origin)


n   Indirectly assign values to symbols

n   Reset the location counter to the specified value

F ORG value

n   Value can be: ¬ constant, ­ other symbol, ® expression

n   No forward reference

n   Example

u SYMBOL: 6bytes
uVALUE: 1word

SYMBOL



VALUE

FLAGS

STAB








uFLAGS: 2bytes








(100 entries)

















u LDA    VALUE, X










.



.

.





















.
.
.


.
.
.

40
ORG Example


n  Using EQU statements
u STAB        RESB 1100

u SYMBOL   EQU  STAB
u VALUE EQU STAB+6 u FLAG EQU STAB+9
n   Using ORG statements

u STAB   RESB 1100

u              ORG  STAB

u SYMBOL                         RESB 6

u VALUE RESW 1

u FLAGS RESB 2

u              ORG  STAB+1100

41
Expressions


n   Expressions can be classified as absolute expressions or relative expressions
u MAXLEN                                                          EQU           BUFEND-BUFFER
u  BUFEND and BUFFER both are relative terms, representing addresses within the program

u However the expression BUFEND-BUFFER represents an absolute value

n     When relative terms are paired with opposite signs, the dependency on the program starting address is canceled out; the result is an absolute value






42
SYMTAB


n   None of the relative terms may enter into a multiplication or division operation

n   Errors:

u BUFEND+BUFFER

u 100-BUFFER

u 3*BUFFER

n   The type of an expression

u 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




43
Example 2.9




SYMTAB
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










LITTAB
C'EOF'
454F46
3
002D
X'05'
05
1
1076



44
Program Blocks

n   Program blocks

u refer to segments of code that are rearranged within a single object program unit
u USE  [blockname]

u At the beginning, statements are assumed to be part of the unnamed (default) block

u If no USE statements are included, the entire program belongs to this single block

u Example: Figure 2.11

u Each program block may actually contain several separate segments of the source program

45
Program Blocks - Implementation


n   Pass 1

u each program block has a separate location counter

u each label is assigned an address that is relative to the start of the block that contains it

u at the end of Pass 1, the latest value of the location counter for each block indicates the length of that block

u the assembler can then assign to each block a starting address in the object program

n   Pass 2

u 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




46
Figure 2.12


n       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








n   For absolute symbol, there is no block number

u line 107

n   Example

u 20                      0006           0                           LDA             LENGTH                                                032060

u LENGTH=(Block 1)+0003= 0066+0003= 0069

u LOCCTR=(Block 0)+0009= 0009

47
Program Readability


n   Program readability

u No extended format instructions on lines 15, 35, 65

u No needs for base relative addressing (line 13, 14)

u LTORG is used to make sure the literals are placed ahead of any large data areas (line 253)

n   Object code

u It is not necessary to physically rearrange the generated code in the object program

u see Fig. 2.13, Fig. 2.14




48
Control Sections and Program Linking

n  Control Sections
are most often used for subroutines or other
u

logical subdivisions of a program

u the programmer can assemble, load, and manipulate each of these control sections separately

u 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
u

for linking control sections together
Fig. 2.15, 2.16
u


49
External Definition and References

n  External definition

uEXTDEF                             name [, name]
u EXTDEF names symbols that are defined in this control section and may be used by other sections

n   External reference

u EXTREF name [,name]

u EXTREF names symbols that are used in this control section and are defined elsewhere

n   Example
u 15
0003 CLOOP
+JSUB
RDREC
4B100000
u 160
0017

+STCH
BUFFER,X
57900000
u 190
0028
MAXLEN
WORD
BUFEND-BUFFER   000000












50
Implementation


n   The assembler must include information in the object program that will cause the loader to insert proper values where they are required

n   Define record
u Col. 1         D

u Col. 2-7 Name of external symbol defined in this control section

u Col. 8-13Relative address within this control section (hexadeccimal)

u Col.14-73 Repeat information in Col. 2-13 for other external symbols

n   Refer record
Col. 1              R
u

u Col. 2-7 Name of external symbol referred to in this control section

u  Col. 8-73Name of other external reference symbols


51
Modification Record
n        Modification record

u Col. 1         M
u Col. 2-7 Starting address of the field to be modified (hexiadecimal)

u Col. 8-9 Length of the field to be modified, in half-bytes (hexadeccimal)

u Col.11-16 External symbol whose value is to be added to or subtracted from the indicated field

u  Note: control section name is automatically an external symbol, i.e. it is available for use in Modification records.

n   Example

u Figure 2.17

u M00000405+RDREC

u M00000705+COPY




52
External References in Expression

n   Earlier definitions

u required all of the relative terms be paired in an expression (an absolute expression), or that all except one be paired (a relative expression)

n   New restriction

u Both terms in each pair must be relative within the same control section

u Ex: BUFEND-BUFFER

u Ex: RDREC-COPY

n   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.




53

Assembler Design Options


















One-pass assemblers

Multi-pass assemblers

Two-pass assembler with overlay structure






54
Two-Pass Assembler with Overlay
Structure
n   For small memory

u pass 1 and pass 2 are never required at the same time

u three segments

F root: driver program and shared tables and subroutines

F pass 1

F pass 2
tree structure
u

uoverlay program





55
One-Pass Assemblers


n   Main problem

u forward references

F data items

F labels on instructions

n   Solution

u data items: require all such areas be defined before they are referenced

u labels on instructions: no good solution











56
One-Pass Assemblers


n   Main Problem

u forward reference

F data items

F labels on instructions

n   Two types of one-pass assembler

u load-and-go

F produces object code directly in memory for immediate execution

u the other

F produces usual kind of object code for later execution

57
Load-and-go Assembler


n   Characteristics

u Useful for program development and testing
Avoids the overhead of writing the object
u

program out and reading it back

u Both one-pass and two-pass assemblers can be designed as load-and-go.

u However one-pass also avoids the over head of an additional pass over the source program

u For a load-and-go assembler, the actual address must be known at assembly time, we can use an absolute program

58
Forward Reference in One-pass Assembler
n   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



59
Load-and-go Assembler (Cont.)


n   At the end of the program

u any SYMTAB entries that are still marked with * indicate undefined symbols

u search SYMTAB for the symbol named in the

END statement and jump to this location to begin execution

n    The actual starting address must be specified at assembly time

n   Example

u Figure 2.18, 2.19






60
Producing Object Code

n   When external working-storage devices are not available or too slow (for the intermediate file between the two passes

n   Solution:

u When definition of a symbol is encountered, the assembler must generate another Tex record with the correct operand address
u The loader is used to complete forward references that could not be handled by the assembler

u The object program records must be kept in their original order when they are presented to the loader

n   Example: Figure 2.20







61
Multi-Pass Assemblers


n   Restriction on EQU and ORG

u no forward reference, since symbols’ value can’t be defined during the first pass

n   Example

u Use link list to keep track of whose value depend on an undefined symbol

n   Figure 2.21
















62
Posted by at