2007-1-30cps4200 system programming 2007 spring 1 systems programming chapter 2 assembler i

2007-1-30 CPS4200 System Programming 2007 Spring

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Systems Programming

Chapter 2 Assembler

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2. Assembler

• Functions of assembler:– Translating mnemonic operation codes

(assembly codes) to their machine language equivalent; and

– Assigning machine addresses to symbolic labels used by the programmer.


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2.1 Basic Assembler Function

• In addition to mnemonic machine instructions, we have the following assembler directive:START Specify name and starting address for the programEND Indicate the end of the source program and (optionally) specify the first executable instruction in the programBYTE Generate character or hexadecimal constant,

occupying as many bytes as needed to represent the constant

WORD Generate one-word integer constantRESB Reserve the indicated number of bytes for a data areaRESW Reserve the indicated number of words of a data area


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2.1 Basic Assembler Function

• See Figure 2-1.


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2.1.1 A Simple SIC Assembler

• Figure 2-2 is the generated object code from Figure 2-1.– Loc: machine address– The program starts at address 1000


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• Translation functions:1. Convert mnemonic operation codes to their

machine language equivalents.

2. Convert symbolic operands to their equivalent machine addresses

3. Build the machine instructions in the proper format

4. Convert the data constants specified in the source program into their internal machine representations

5. Write the object program and the assembly listing


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• Translation f unction #2 presents a problem:10 1000 First STL RETADR 141033

RETADR’s address is not known yet when we process the translation!!

• forward reference – a reference to a label (RETADR) that is defined later in the program.

• Solution: Two passes compilation – First pass scan the source program for label definition

s and assign addresses such as those in Loc column in Figure 2.2.


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• In addition to translating the instructions of the source program, the assembler must process statements called assembler directive.

– They provide instruction to the assembler itself.

– Example, BYTE, WORD, RESB, RESW, START, END.


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• Finally, the assembler must write the generated object code onto some output device.

– Three types of records: Header, Text, and End.• Header: contains the program name, starting

address, and length. • Text records: contains the translated

instructions and data of the program, together with an indication of the addresses where these are to be loaded.

• End Record: marks the end of the object program and specifies the address in the program where execution is to begin.


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Header record:

Col.1 H

Col. 2-7 Program name

Col. 8-13 Starting address of object program (Hex)

col. 14-19Length of object (Hex)program in bytes


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Text record:

col.1 T

col. 2-7 Starting address for object code in this record (Hex)

col. 8-9 Length of object code in this record in bytes (Hex)

col. 10-69Object code, represented in hexadecimal


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End record:

col.1 E

col. 2-7 address of first executable instruction in object program


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• Pass1 (define symbols)– Assign addresses to all statements in the

program– Save the values (addresses) assigned to all

labels for use in Pass 2.– Perform some processing of assembler

directives. (This includes processing that effects address assignment, such as determining the length of data areas defined by BYTE, RESW, etc.)


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• Pass2 (assemble instructions and generate object program)

– Assemble instructions. (translating operation codes and looking up addresses.)

– Generate data values defined by BYTE, WORD, etc.

– Perform processing of assembler directive not done during Pass 1.

– Write the object program and the assembly listing.

2007-1-30cps4200 system programming 2007 spring 1 systems programming chapter 2 assembler i

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