2.2Machine-Dependent Assembler Features

Machine-Dependent Assembler Features

Machine dependent assembler features z

z

Instruction formats and addressing modes Program relocation

• Generate an object program that can be placed anywhere in the physical memory.

Machine-dependent assembler features

2

1

Review of instruction formats 8

z

SIC Format

z

Format 1 (1 byte)

z

Format 2 (2 bytes)

z

Format 3 (3 bytes)

z

Format 4 (4 bytes)

1

15

opcode x

address

8 op

SIC/XE

8

4

4

op

r1

r2

6 op 6 op

1 11 1 1 1

12

n i x b p e disp 1 11 1 1 1

20

n i x b p e address


3

Review of instruction formats (cont’d) z

Register-to-register instructionsop r1,r2

• z

Register-to-register instructions are shorter and do not require another memory reference. Use register-toregister 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. Machine-dependent assembler features

4

2

Review of addressing modes z

Immediate addressing:

• • z

Indirect addressing:

• • z

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

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.

z

• The assembler directive BASE is used in conjunction with base relative addressing. Index addressing: op m,X

z

Direct addressing:

•

160

STCH

BUFFER,X


5

Addressing modes for SIC/XE Flag bits p bit=1: program-counter relative addressing mode e bit=1: extended instruction format Assembler language notation c indicates a constant between 0 and 4095. m indicates a memory address or a constant value larger than 4095. Note 4 Format 4 instruction D Direct-addressing instruction A Assembler selects either programcounter relative or base-relative mode S Compatible with instruction format for standard SIC machine Machine-dependent assembler features

6

3

“START 0” z

It indicates a relocatable program.

• The program will be translated exactly as if it were to be loaded at machine address 0.


7

Register-to-register instructions z

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.). 125 RDREC CLEAR X B410

1 Machine-dependent assembler features

8

4

Register-to-memory instructions z

Most of register-to-memory instructions are assembled using either

• • z

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.

• 15

The programmer must specify the 4-byte format by adding the prefix + to the operation code in the source statement. CLOOP

125 RDREC

+JSUB

RDREC

CLEAR

X


4B101036

9

Register-to-memory instructions (cont’d) z

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.


10

5

Register-to-memory instructions (cont’d) 15

0006

CLOOP

125

1036

RDREC

6

1

1

+JSUB RDREC ... CLEAR X

1

1

1

1

4B101036 B410

20

op

n

i

x

b

p

e

address

010010

1

1

0

0

0

1

0000,0001,0000,0011,01 10

4B101036


11

A SIC/XE program (Fig. 2.5) An assembler directive

a reloctable program

4-byte instruction

immediate operand

indirect addressing


12

6

A SIC/XE program (cont’d)

register-toregister instruction


13

A SIC/XE program (cont’d)


14

7

Program from Fig 2.5 with object code


15

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


16

8

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

17


Calculation for register-toregister instruction 125

1036

RDREC

CLEAR

X

8

4

4

op

r1

r2

10110100

0001

0000

B410

B410 150

1049

COMPR

A,S

8

4

4

op

r1

r2

10100000

0000

0100

A004

A004 Machine-dependent assembler features

18

9

Displacement calculation for programcounter relative addressing z z

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 0000 FIRST

STL

95 0030

RETADR

• • •

RETADR ... RESW 1

17202D

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.

6 op 000101

1 n 1

1 1 i x 1 0

1 b 0

1 p 1

1 e 0

12 disp 000000101101

17202D

19


Displacement calculation for programcounter relative addressing (cont’d) 16 0006CLOOP+JSUB 40 0017

RDREC ... CLOOP

J

4B101036 3F2FEC

disp = TA-(PC) = 0006-001A= -0014 = (111111101100)2 using 2’s complement for negative numbers 6 op 001111

1 n 1

1 1 i x 1 0

1 b 0

1 p 1

1 e 0

12 disp 111111101100

3F2FEC Machine-dependent assembler features

20

10

Displacement calculation for base relative addressing z z z

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

12

•

LENGTH

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

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.


21

Displacement calculation for base relative addressing (cont’d) 1050036 BUFFER 160104E

RESB STCH

4096 BUFFER,X

57C003

1. Try indexed and program-counter relative addressing disp = TA – (PC)=0036 – 1050 = -101A16<-2048—out of range 2. Try indexed and base relative addressing disp = TA – (BASE)=0036 – 0033 =000316 6 op 0101,0 1

1 n 1

1 1 i x 1 1

1 b 1

1 p 0

1 e 0

12 disp 0000,0000,0011

57C003 Machine-dependent assembler features

22

11

Calculation for immediate addressing z

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

55

0020 6 op 000000

LDA 1 n 0

1 1 i x 1 0

1 b 0

#3

1 p 0

1 e 0

010003

12 disp 000000000011

010003 133

103C

+LDT

6

1

1

1

1

#4096 75101000 1

1

20

op

n

i

x

b

p

e

address

011101

0

1

0

0

0

1

0000,0001,0000,0000,00 00

75101000


23

Calculation for immediate addressing (cont’d) the value of the symbol LENGTH is its address. 20

0003

100

0033

LDB #LENGTH ... RESW 1

LENGTH

69202D

Line 20 combines the program-counter relative addressing with immediate addressing. disp = TA – (PC)=0033-0006=002D 6 op 011010

1 n 0

1 1 i x 1 0

1 b 0

1 p 1

1 e 0

12 disp 0000,0010,1101

69202D Machine-dependent assembler features

24

12

Calculation for indirect addressing 70

002A

95

0030

J

@RETADR ... RESW 1

RETADR

3E2003

Line 70 combines the program-counter relative addressing with indirect addressing. disp = TA-(PC)=0030-002D=0003 6 1 1 1 1 1 op 001111

n 1

i x 0 0

b 0

p 1

1 e 0

12 disp 0000,0000,0011

3E2003 Machine-dependent assembler features

25

Program relocation z

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. z

The task of adjusting object programs so they may be placed in arbitrary memory locations is called relocation. Machine-dependent assembler features

26

13

Relocation problem No matter where the program is loaded, RDREC is always 1036 bytes past the starting address of the program. Solution must be modified •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. Machine-dependent assembler features

27

What parts of the program need to be modified? z

z

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. Machine-dependent assembler features

28

14

Relocatable program z

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.


29

The modification record z

z

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.


30

15

Modification record format Col. 1 Col. 2-7 Col. 8-9

M Starting location of the address field to be modified, relative to the beginning of the program Length of the address field to be modified, in halfbytes If the field contains an odd number of half-bytes, the starting location begins in the middle of the first byte.

15 0006 CLOOP +JSUB M00000705 35 0013 +JSUB M00001405 65 0026 +JSUB M00002705

RDREC

4B101036

WRREC

4B10105D

WRREC

4B10105D


31

Object program with modification record relocatable

same as the absolute program with starting address 0 modification records Machine-dependent assembler features

32

16

2.2Machine-Dependent Assembler Features

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