2.2Machine-Dependent Assembler Features

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Machine-Dependent Assembler Features

Machine-dependent assembler features

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Machine dependent assembler features

Instruction formats and addressing modes Program relocation• Generate an object program that can be

placed anywhere in the physical memory.

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Review of instruction formats

SIC Format

Format 1 (1 byte)

Format 2 (2 bytes)

Format 3 (3 bytes)

Format 4 (4 bytes)

r2r1op448

dispepbxinop

121111116

addressepbxinop

201111116

opcode x address

8 1 15

op8 SIC/XE


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Review of instruction formats (cont’d)

Register-to-register instructionsop r1,r2• Register-to-register instructions are shorter and do not

require another memory reference. Use register-to-register instructions instead of register-to-memory instructions whenever possible.

Extended format (4 bytes): +op• If the displacements required for both program-counter

relative and base relative addressing are too large to fit into a 3-byte instruction, then the 4-byte extended format must be used.

• The programmer has to specify this form of addressing when it is required.

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Review of addressing modes

Immediate addressing: op #c; op #m• Using the immediate addressing, the operand is already present as part of the

instruction and need not be fetched from anywhere.• 55 LDA #3

Indirect addressing: op @m; op @c• The use of indirect addressing often avoids the need for another instruction.• 70 J @RETADR

PC-relative or Base-relative addressing: • Instructions that refer to memory are normally assembled using either

• the program-counter relative mode or • the base relative mode.

• The assembler directive BASE is used in conjunction with base relative addressing.

Index addressing: op m,X• 160 STCH BUFFER,X

Direct addressing:


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Addressing modes for SIC/XEFlag bits

p bit=1: program-counter relative addressing mode

e bit=1: extended instruction format

Assembler language notationc indicates a constant between 0 and 4095.m indicates a memory address or a

constant value larger than 4095.

Note4 Format 4 instructionD Direct-addressing instructionA Assembler selects either program-

counter relative or base-relative modeS Compatible with instruction format for

standard SIC machine

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“START 0”

It indicates a relocatable program.• The program will be translated exactly as if it

were to be loaded at machine address 0.


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Register-to-register instructions

The translation of register-to-register instructions must convert the register mnemonics to numbers. • The register mnemonics can be stored in a predefined

table, or• in the SYMTAB.

• SYMTAB would be preloaded with the register names (A,X, etc.) and their values (0, 1, etc.).

125RDREC CLEAR X B410

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Register-to-memory instructionsMost of register-to-memory instructions are assembled using either • program-counter relative addressing or

• TA=(PC)+displacement (12-bit) • The displacement must be between –2048 and +2047.

• base relative addressing.• TA=(BASE)+displacement (12-bit) • The displacement must be between 0 to 4095.

If the required displacement is too large, then the 4-byte extended instruction format must be used.• The programmer must specify the 4-byte format by adding the

prefix + to the operation code in the source statement.15 CLOOP +JSUB RDREC 4B101036125 RDREC CLEAR X


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Register-to-memory instructions (cont’d)

If extended format is not specified, the assembler may first attempt to translate the instruction using program-counter relative addressing.

If the required displacement is out of range, the assembler then attempts to use base relative addressing.

If neither form of relative addressing is applicable, then the instruction cannot be properly assembled and the assembler must generate an error message.

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Register-to-memory instructions (cont’d)

15 0006 CLOOP +JSUB RDREC 4B101036 ...

125 1036 RDREC CLEAR X B410

201111116

addressepbxinop

0000,0001,0000,0011,0110

100011010010

4B101036


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A SIC/XE program (Fig. 2.5)

indirectaddressing

immediate operand

An assembler directive

4-byte instruction

a reloctable program

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A SIC/XE program (cont’d)

register-to-register

instruction


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A SIC/XE program (cont’d)

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Program from Fig 2.5 with object code


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Program from Fig 2.5 with object code (cont’d)

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Program from Fig 2.5 with object code (cont’d)


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Calculation for register-to-register instruction

125 1036 RDREC CLEAR X B410

150 1049 COMPR A,S A004

r2r1op448

0000000110110100

B410

r2r1op448

0100000010100000

A004

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Displacement calculation for program-counter relative addressing

TA = (PC)+disp => disp = TA – (PC)The program counter is advanced after each instruction is fetched and before it is executed.

• PC will contain the address of the next instruction.10 0000FIRST STL RETADR 17202D

...95 0030 RETADR RESW 1

• After fetching the instruction in line 10, PC becomes 0003. Hence, disp=0030-(PC)=0030-0003=002D.

• p bit is set to 1 to indicate program-counter relative addressing.• Bits n and i are both set to 1, indicating neither indirect nor immediate

addressing.

dispepbxinop121111116

00000010110101001100010117202D


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Displacement calculation for program-counter relative addressing (cont’d)

16 0006CLOOP+JSUB RDREC 4B101036...

40 0017 J CLOOP 3F2FEC

disp = TA-(PC) = 0006-001A= -0014 = (111111101100)2


1111111011000100110011113F2FEC

using 2’s complement for negative numbers

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Displacement calculation for base relative addressing

TA=(BASE)+disp=>disp = TA-(BASE)To compute displacements, the assembler must know the content of register B that must be specified by the programmer.The assembler directive “BASE” tells the assembler the content of the base register B.• 13 BASE LENGTH

• This statement informs the assembler that the base register will contain the address of LENGTH.

• 12 LDB #LENGTH• This statement stores the address of LENGTH to register B.

• When register B is used for another purpose, the programmer can use the assembler directive “NOBASE” to inform the assembler that the content of the base register can no longer be relied upon for addressing.


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Displacement calculation for base relative addressing (cont’d)

1050036 BUFFER RESB 4096160104E STCH BUFFER,X 57C003


0000,0000,00110011110101,01 57C003

1. Try indexed and program-counter relative addressingdisp = TA – (PC)=0036 – 1050 = -101A16<-2048—out of range2. Try indexed and base relative addressingdisp = TA – (BASE)=0036 – 0033 =000316

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Calculation for immediate addressing

Convert the immediate operand to its internal representation and insert it into the instruction.

55 0020 LDA #3 010003

133 103C +LDT #4096 75101000


000000000011000010000000010003

201111116

addressepbxinop

0000,0001,0000,0000,0000

100010011101

75101000


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Calculation for immediate addressing (cont’d)

20 0003 LDB #LENGTH 69202D...

100 0033 LENGTH RESW 1

Line 20 combines the program-counter relative addressing with immediate addressing.

disp = TA – (PC)=0033-0006=002D


0000,0010,110101001001101069202D

the value of the symbol LENGTH is its address.

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Calculation for indirect addressing

70 002A J @RETADR 3E2003...

95 0030 RETADR RESW 1

Line 70 combines the program-counter relative addressing with indirect addressing.

disp = TA-(PC)=0030-002D=0003dispepbxinop

121111116

0000,0000,00110100010011113E2003


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Program relocation

The object program cannot be assigned to a fixed address.• To take full advantage of hardware, we must

be able to load programs into memory wherever there is room for it.

The task of adjusting object programs so they may be placed in arbitrary memory locations is called relocation.

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Relocation problem

must be modified

No matter where the program is loaded, RDREC is always 1036 bytes past the starting address of the program.

Solution

•The assembler insert the address of RDREC relative to the start of the program.

•The assembler also produces a command for the loader, instructing it to add the beginning address of the program to the address field in the JSUB instruction at load time.


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What parts of the program need to be modified?

The parts of the object program that require modification at load time are those that specify direct addresses.No modifications are needed for the operands • that are not memory references, or• specified by

• the program-counter addressing, and• base relative addressing.Relocatable is one advantage of relative addressing.

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Relocatable program

The assembler can identify for the loader those parts of the object program that need modification.• An object program that contains the

information necessary to perform this kind of modification is called a relocatable program.


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The modification record

The assembler produces a modification record describing the address and length of an address field to be modified.The loader will add the beginning address of the loaded program to the address field specified by a modification record.

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Modification record formatCol. 1 MCol. 2-7 Starting location of the address field to be modified,

relative to the beginning of the program Col. 8-9 Length of the address field to be modified, in half-

bytesIf the field contains an odd number of half-bytes, the startinglocation begins in the middle of the first byte.

15 0006 CLOOP +JSUB RDREC 4B101036M0000070535 0013 +JSUB WRREC 4B10105DM0000140565 0026 +JSUB WRREC 4B10105DM00002705


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Object program with modification record

relocatable

same as the absolute program with starting address 0

modification records

Documents

2.2Machine-Dependent Assembler Features