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

- Forward references table (FRT): 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

Forward references table

String storage buffer

Partially configured object file

Figure 2. A simple two pass assembler.

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

Symbol Table

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

Forward References Table

SSB

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

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.

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 ? Forward References 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

Forward References Done

Table

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

Figure 5. Second pass of a two-pass assembler.

Date: 2015-01-11; view: 1608