assembler tables

8/12/2019 Assembler Tables

http://slidepdf.com/reader/full/assembler-tables 1/28

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 (also called as mnemonic code) and produces its machine language

equivalent (object code) along with the information for the loader.

Assembly Language Program

___________________

______________

_______________

Assembler Linker exe

Executable program generation from an assembly source code



Assembler languages-structure

• Label

– symbolic labeling of an assembler address (command address at Machine level)

• Mnemomic

– Symbolic description of an operation

• Operands

– Contains of variables or addresse if necessary

• Comments

– ignored by assembler

– used by humans to document/understand programs

– tips for useful comments:

• avoid restating the obvious, as ‚decrement R1‛

• provide additional insight, as in ‚accumulate product in R6‛

<Mnemomic> <Operand> Comments<Label>



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)



DESIGN OF ASSEMBLER

•Assembler are designed using 2 – passes.

•The first pass defines the symbols and the literals

•The second pass generates the instructions and addresses

Pass 1 Pass 2Intermediate

fileObject

codes

Sourceprogram

OPTAB SYMTAB SYMTAB



ASSEMBLER PROCESS

ASSEMBLY

LANGUAGE

PROGRAM

PASS1 PASS2EXECUTABLE

IMAGE

SYMBOL

TABLE

First Pass:

•scan program file

•find all labels and calculate the corresponding addresses; this is

called the symbol table

Second Pass:

•convert instructions to machine language, using information from

symbol table



General Design Procedure of Two Pass

Assembler

1. Specify the problem

2. Specify data structures

3. Define format of data structures

4. Specify algorithm

5. Look for modularity [capability of one program to be

subdivided into independent programming units.]6. Repeat 1 through 5 on modules.



JOHN

FOUR

FIVE

TEMP

START

USING

L

AST

DC

DC

DS

END

0

*, 15

1, FIVE

1, FOUR1, TEMP

F ‘4’

F ‘5’

1F

STATEMENT OF PROBLEM

EX:



FIRST PASS SECOND PASSRELATIVE

ADDRESS

(LC)

MNEMONIC

INSTRUCTION

RELATIVE

ADDRESS

MNEMONIC

INSTRUCTION

JOHN START 0

USING 8, 15

L 1, FIVE 0 L 1, -(0,15) 0 L 1, 16(0,15)

A 1, FOUR 4 A 1, -(0,15) 4 A 1, 12(0,15)

ST 1, TEMP 8 ST 1, -(0,15) 8 ST 1, 20(0,15)

FOUR DC F '4' 12 4 12 4

FIVE DC F '5' 16 5 16 5

TEMP DS 1F 20 - 20 -

Intermediate steps in assembling a program



JOHN START 0

START is a pseudo – op, which indicates the beginning of the program.

JOHN is the name of the program which is passed on to the loader by the assembler.

0 indicates the relative address of this program starts at 0

USING *,15

USING is a pseudo – op, that tells the assembler that register 15 is used as the base

register, and at execution time will contain the address of the first instruction of the

program i.e., 0.

L 1, FIVE

Represents LOAD instruction which is of type RX.

Equivalent mnemonic instruction after pass 1 is

L 1, - (0,15)

Where:

- Represents offset

0 Represents Index Register

15 Represents Base Register



A 1, FOUR

Represents ADD instruction which is of type RX.

The offset(offset usually denotes the number of address locations added to a base

address in order to get to a specific absolute address) of the FOUR is not known at this

point therefore the equivalent mnemonic instructions after pass 1 is:

A 1, - (0, 15)

ST 1, TEMP

Stores the contents of register 1 to TEMP.

It is of RX type instruction

The offset of TEMP is not known therefore the equivalent mnemonic instructions after

pass 1 is:

ST 1, -(0, 15)



FOUR DC F ‘4’

DC is a pseudo – op which defines a constant value 4 for FOUR.

It is been stored at relative locations 12.

One full word is reserved for this constant, therefore location counter is incremented by 4

FIVE DC F ‘5’

TEMP DS 1F

DS is a pseudo op which defines storage space for TEMP.

One full word is reserved for TEMP at relative location 20.

The Second Pass evaluates the fields and generate, the codes. i.e., it goes through the

program and fills the offset values.



Specify the problem

Pass1: Define symbols & literals.

1) Determine length of m/c instruction [MOTGET1]

2) Keep track of Location Counter [LC]

3) Remember values of symbols until pass 2 [STSTO]

4) Process some pseudo ops[EQU,DS etc] [POTGET1]

5) Remember Literals [LITSTO]

Outline of steps involved in pass 1 is given the following

flowchart.



Pass2: Generate object program

1) Look up value of symbols [STGET]

2) Generate instruction [MOTGET2]

3) Generate data (for DS, DC & literals)

4) Process pseudo ops [POTGET2]

An outline of the steps involved in pass 2 is given

in the following flowchart



Step 2. Data structure:-

Pass1: Databases

1. Input source program

2. ‚LC‛ location counter used to keep track of each instructions location.

3. M/c operation table (MOT) [indicates Symbolic mnemonic for each instruction & its

length(2, 4, 6 bytes)]

4. Pseudo operation table [POT], [indicates Symbolic mnemonic & action to be taken for

each pseudo op in pass 1]5. Symbol Table (ST) to store each label & it’s corresponding value.

6. Literal Table (LT), to store each literal (variable) encountered & it’s corresponding

assigned location.

7. Copy of input to used later by PASS-2. This may be stored in a secondary storage device,

such as magnetic tape, disk as drum, or the original sources may be read by the assembler

a second time for pass 2.



• Pass2: Databases

1. Copy of source program input to Pass1.

2. Location Counter (LC)

3. MOT [Mnemonic, length, binary m/c op code, etc.] that indicates for each instruction,

a. Symbolic mnemonic

b. Lengthc. Binary machine op code

d. Format (eg: R, S, RX, SI)

4. POT [indicates for each pseudo op the Mnemonic & action to be taken in Pass2]

5. ST [prepared by Pass1, containing each label & its corresponding value]

6. Base Table [or register table] indicates which registers are currently specified using ‘USING’ pseudo op

& what are contents.7. Literal table prepared by Pass1. [Lit name & value].

8. A work – space, INST, that is used to hold each instruction as its various parts (eg: binary op-code,

registers fields, length fields, displacement fields) are being assembled together.

9. A work space, PRINT LINE used to produce a printed listing.

10. A work space, PUNCH CARD, used prior to actual outputting for converting the assembled instructions

into the format needed by the loader.

11. An output deck of assembled instructions in the format needed by the loader.



DATABASES USED BY PASS1 AND PASS2

Base Table(BT)



Format of Data Structures

• Machine Operation Table

– The op-code is the key and it’s value is the binary op code

equivalent, which is used for use in generating machine code.

– The instruction length is stored for updating the location

counter.

– Instruction format is use in forming the m/c language

equivalent



Pseudo Operation Table

• It is a fixed table that contains the pseudo – ops (symbolic

mnemonics) and the corresponding actions.

• The POT for pass 1 and pass 2 are the same and it contains the

name and address.• Size of POT is 8 bytes per entry.

• Each pseudo – op is listed with an associated point to the

assembler routine for processing the pseudo – op.



ST Symbol Table

• It is a variable table that contains labels and its value.

•ST for pass1 & pass2 are the same and contains symbol, value, length andrelocation fields.

• Size of table is 14 bytes per entry.

• The length field indicates the length in bytes of the instruction to which symbol

is attached.

• It is used by the assembler to calculate the length codes used with certain SS

type instructions.

Ex:

• H DS F

the length of H is 4, since the size of 1 full word is 4 bytes.

• AC EQY 2

the length of AC is 1 byte, because if a symbol is equivalent to another,

its length is made the same as that of the other.



The relative location indicator tells the assembler whether the value of the symbol is absolute (A)

[i.e., the value of the symbol does not change if the program is moved in the core] or relative (R) to

the base of the program.

Ex:

•AC EQU 2 The relocation field of AC is A

• FOUR DC F ‘4’ The relocation field of FOUR is R



LT Literal Table

• It is a variable table that contains literals and its value.

•It is same like symbol table, instead of symbols, here we have literals.

Ex:

A 1, = F ‘4’

The literal table entries for this is as follows:

Symbol (8 bytes)

(characters)

Value (4 bytes)

Hexadecimal

Length (1 byte)

Hexadecimal

Relocation (1 byte)

(character)

F ‘4’ 4 04 “R”



BT Literal Table

• It is used by the assembler to generate the proper base register reference in

machine instructions and to compute the correct offsets.

• In the assembly process, the assembler must generate an address, which

contains the offset, a base register number and an index register number.

• When generating an address, the assembler may use the base register table to

choose a base register that will contain a value closest to the symbolic

reference.

Base Register Number = the base register containing a value closest to the

symbolic reference.Offset = value of symbol in symbol table – contents of base

register.



SLNO

NAME OF

THE

TABLE

TYPE OF THETABLE

PASS 1ENTRIES

PASS 2ENTRIES

SIZE

1 MOT FIXEDMnemonic Opcode,

length

Mnemonic opcode,

length and format6 bytes per entry

2 POT FIXED TABLEPseudo - op and

address

Pseudo - op and

address8 bytes per entry

3 ST VARIABLEsymbol, value length

and relocations

Symbol, value length

and relocation

14 bytes per

entry

4 LT VARIABLETABLE

Literals, value lengthand relocations

literal, value lengthand relocations

14 bytes perentry

5 BTVARIABLE

TABLE

Base register number and

contents of base register4 bytes per entry

The following table summarizes the fixed and variable tables used by the 2 – pass

assembler

assembler tables

Documents