Assembler Notes 5s1m6x

Yeditepe University Department of Computer Engineering

CSE 232 Systems Programming Lecture Notes #2 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)

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Assembler: An assembler does the following: 1. Generate machine instructions - evaluate the mnemonics to produce their machine code - 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 Assembler A two- assembler performs two sequential scans over the source code: 1: symbols and literals are defined 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 language program

1

2

Symbol table Configuration table String storage buffer Partially configured object file

machine language program

Figure 2. A simple two assembler.

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Example 1: Decrement number 5 by 1 until it is equal to zero. assembly language program ----------------------START 0100H LDA #5 LOOP: SUB

#1

COMP #0 JGT

LOOP

RSUB

memory address

object code in memory

0100 0101 0102 0103 0104 0105 0106 0107 0108 0109 010A 010B 010C 010D 010E

01 00 05 1D 00 01 29 00 00 34 01 03 4C 00 00

placed in 1

END

RSUB

Op-code Table Mnemonic LDA SUB COMP LDX ADD TIX JLT JGT

Addressing mode immediate immediate immediate immediate indexed direct direct direct

Opcode 01 1D 29 05 18 2C 38 34

implied

Symbol Table Symbol LOOP

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Value 0103

4C

Example 2: Sum 6 elements of a list which starts at location 200. assembly language program ----------------------START 0100H LDA #0 LDX LOOP: ADD TIX JLT

#0 LIST, X COUNT LOOP

RSUB LIST:

WORD 200

COUNT: WORD 6

memory address

object code in memory

0100 0101 0102 0103 0104 0105 0106 0107 0108 0109 010A 010B 010C 010D 010E 010F 0110 0111 0112 0113 0114 0115 0116 0117

01 00 00 05 00 00 18 01 12 2C 01 15 38 01 06 4C 00 00 00 02 00 00 00 06

END Symbol Table Symbol LOOP LIST COUNT

Address 0106 0112 0115

Configuration Table Offset 0007 000A

DC00 DC01 DC02 DC03 DC04 DC05

SSB pointer for the symbol DC00 DC05 SSB 4CH 49H 53H 54H 5EH

ASCII for L,I,S,T

ASCII for separation character 4

placed in 2 placed in 2 placed in 1

1 • 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

2

N N

pseudo-op

Y

what kind?

N Label

EQU

WORD/ BYTE

RESW/RESB

Y N Enter label in ST

Label

N

Label

Enter label in ST Y Enter label in ST

Y Enter label in ST

Call translator Place constant in machine code Advance LC

Advance LC by the number of bytes specified in the pseudo-op

Figure 3. First of a two- assembler.

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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 will be needed in 2 ? N

Y Set up an entry in Configuration Table

Advance LC by the number of bytes in op-code table return

Figure 4. Flowchart of a translator routine 2 - Fills addresses and data that was unknown during 1. 1 More lines in Configuration Table

N Done

Y Get the next line Retrieve the name of the symbol from SSB Get the value of the symbol from ST Compute the location in memory where this value will be placed (starting address + offset) Place the symbol value at this location

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Figure 5. Second of a two- 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 program ----------------------START LDA #0 LDX LOOP: ADD

#0 LIST, X

TIX

COUNT

JLT

LOOP

RSUB LIST:

WORD 200

COUNT: WORD 6

memory address

object code in memory

0000 0001 0002 0003 0004 0005 0006 0007 0008 0009 000A 000B 000C 000D 000E 000F 0010 0011 0012 0013 0014 0015 0016 0017

01 00 00 05 00 00 18 00 12 2C 00 15 38 00 06 4C 00 00 00 02 00 00 00 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. 7

One- 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- Assemblers Make as many es as needed to process the definitions of symbols. Example 3: A B C

EQU B EQU C DS 1

3 es are required to find the address for A.

Such references can also be solved in two es: 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 B C D

EQU EQU EQU DS

B D D 1

At the end of 1: Symbol Table A &1 B B &1 D C &1 D D 200

0 A

0

0 B

C

0

After evaluating dependencies: Symbol Table A 200 B 200 C 200

0 0 0 8

D

200

0

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