UNIT-IIILoaders and Linkers

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Loaders

• A loader is a system program that performs the loading function.– many also support relocation & linking– others have a separate linker and loader

• Basic Functions– bringing an object program into memory– starting its execution

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Input• Object program:

– contains translated instructions and data from the source program.

– specifies addresses in memory where these items are to be loaded.

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Basic Functions

• Allocation: allocate space in memory for the programs

• Linking: Resolve symbolic references between object files– combines two or more separate object programs – supplies the information needed to allow

references between them

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Basic Functions

• Relocation: Adjust all address dependent locations, such as address constants, to correspond to the allocated space– modifies the object program so that it can be

loaded at an address different from the location originally specified

• Loading: Physically place the machine instructions and data into memory

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Basic Functions

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Design of an AbsoluteLoader

• Its operation is very simple– no linking or relocation

• Single pass operation– check H record to verify that correct program has

been presented for loading– read each T record, and move object code into

the indicated address in memory– at E record, jump to the specified address to

begin execution of the loaded program.

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Example of Object Program

H _ COPY 001000 _ 00107A

T _ 001000 _ 1E1 _ 41033 _ 482039 _ 001036 _ 281030 _ 301015 _ 482061 _ 3C1003 _ 00102A _ 0C1039 _ 00102D

T _ 00101E _ 15 _ 0C1036 _ 482061 _ 081033 _ 4C0000 _ 454F46 _ 000003 _ 000000

T _ 002039 _ 1E 041030 _ 001030 _ E0205D _ 30203F _ D8205D _ 281030 _ 302057 _ 549039 _ 2C205E _ 38203F

T _ 002057 _ 1C _ 101036 _ 4C0000 _ F1 _ 001000 _ 041030 _ E02079 _ 302064 _ 509039 _ DC2079 _ 2C1036

T _ 002073 _ 07 _ 382064 _ 4C000 _ 05

E _ 001000

Object Program – each byte of assembled code is given using its hexadecimal

representation in character form– easy to read by human beings

In general– each byte of object code is stored as a single byte – most machine store object programs in a binary form– we must be sure that our file and device conventions do not

cause some of the program bytes to be interpreted as control characters

No text record corresponds here.XXX indicates that the previouscontents of these locations remainunchanged.

Absolute Loader Algorithm

Begin

read Header record

verify program name and length

read first Text record

While record type ≠ ‘E’ do

begin

{if object code is in character form, convert into internal representation}

move object code to specified location in memory

read next object program record

end

jump to address specified in End record

end

Loader Schemes

• Compile and Go– The assembler run in one part of memory– place the assembled machine instructions and

data, as they are assembled, directly into their assigned memory locations

– When the assembly is completed, the assembler causes a transfer to the starting instruction of the program

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Absolute Loader

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Disadvantages

• A portion of memory is wasted because the memory occupied by the assembler is unavailable to the object program.

• It is necessary to re-translate (assemble) the user's program file every time it is run.

• It is very difficult to handle multiple segments, especially if the source programs are in different.

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Disadvantages• If changes were made to MAIN that increased its

length to more than 300 bytes– the end of MAIN (at 100 + 300 = 400) would overlap

the start of SQRT (at 400)– It would then be necessary to assign SQRT to a new

location • changing its START and re-assembling it

• Furthermore, it would also be necessarily to modify all other subroutines that referred to the address of SQRT.

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A Simple Bootstrap Loader• Bootstrap Loader

– When a computer is first tuned on or restarted, a special type of absolute loader, called bootstrap loader is executed

– This bootstrap loads the first program to be run by the computer -- usually an operating system

• Example (SIC/XE bootstrap loader)– The bootstrap itself begins at address 0 – It loads the OS starting address 0x80– No header record or control information, the

object code is consecutive bytes of memory

SIC/XE Bootstrap Loader LogicBeginX=0x80 (the address of the next memory location to be loaded Loop

AGETC (and convert it from the ASCII character code to the value of the hexadecimal digit)

save the value in the high-order 4 bits of SAGETC

combine the value to form one byte A (A+S)store the value (in A) to the address in register XXX+1

End

0~9 : 48 A~F : 65

GETC Aread one characterif A=0x04 then jump to 0x80if A<48 then GETCA A-48 (0x30)if A<10 then returnA A-7 (48+7=55)return

BOOTSTRAP LOADERSHow is the loader itself loaded into the memory ?• When computer is started – with no program in

memory, a program present in ROM ( absolute address) can be made executed – may be OS itself or A Bootstrap loader, which in turn loads OS and prepares it for execution.

• The first record ( or records) is generally referred to as a bootstrap loader – makes the OS to be loaded

• Such a loader is added to the beginning of all object programs that are to be loaded into an empty and idle system

MACHINE-DEPENDENT LOADER FEATURES

Absolute Loader – Simple and efficient

Disadvantage is – programmer has to specify the starting address

One program to run – no problem – not for several

Difficult to use subroutine libraries efficiently

RELOCATION

Execution of the object program using any part of the available and sufficient memory

The object program is loaded into memory wherever there is room for it

The actual starting address of the object program is not known until load time

RELOCATING LOADERS

•Efficient sharing of the machine with larger memory and when several independent programs are to be run together•Support the use of subroutine libraries efficiently

METHODS FOR SPECIFYING RELOCATION

•Use of modification record Refer Figure•Use of relocation bit Refer Figure

•Each instruction is associated with one relocation bit•These relocation bits in a Text record is gathered into bit masks

MODIFICATION RECORD

Modification recordcol 1: Mcol 2-7: relocation address col 8-9: length (halfbyte)col 10: flag (+/-)col 11-17: segment name

•For complex machines•Also called RLD specification

- Relocation and Linkage Directory

HCOPY 000000 001077

T000000 1D17202D69202D48101036…

4B105D3F2FEC032010

T00001D130F20160100030F200D4B10105D3E2003454F46

T001035 1DB410B400B44075101000…

33200857C003B850

T0010531D3B2FEA1340004F0000F1..53C003DF2008B850

T00070073B2FEF4F000005

M00000705+COPY

M00001405+COPY

M00002705+COPY

E000000

Object program with relocation by Modification records

PROGRAM LOGIC FOR PASS 1

• Pass 1: – - Assign addresses to all external symbols

• Variables & Data structures• - PROGADDR (program load address) from OS• - CSADDR (control section address)• - CSLTH (control section length• - ESTAB• Refer Algorithm for Pass 1 of LL in Fig. 3.11(a)

– - Process Define Record

begin

get PROGADDR from operating system

set CSADDR to PROGADDR {for first control section}

while not end of input do

begin

read next input record {Header record for control section}

set CSLTH to control section length

search ESTAB for control section name

if found then set error flag {duplicate external symbol}

else enter control section name into ESTAB with value CSADDR

while record type != ‘E’ do

begin

read next input symbol

if record type = ‘D’ then

for each symbol in the record do

begin

search ESTAB for symbol name

if found then set error flag (duplicate external symbol)

else enter symbol into ESTAB with value (CSADDR + indicated address)

end {for}

end {while != ‘E’}

add CSLTH to CSADDR {starting address for next control section}

end {while not EOF}

Linking Loader - Pass1

(Assigns addresses to all external symbols)

ESTAB = External symbol tablePROGADDR = Program load addressCSADDR = Control section address

PROGRAM LOGIC FOR PASS 2

• Pass 2: – - Perform the actual loading, relocation, and

linking• Modification record

– - Lookup the symbol in ESTAB• End record for a main program

– - Transfer address• Refer Algorithm for Pass 2 of LL in Fig. 3.11(b)

– - Process Text record and Modification record

Begin

Set CSADDR to PROGADDR

Set EXECADDR to PROGADDR

While not end of input do

begin

read next input record {Header record}

set CSLTH to control section length

while record type != ‘E’ do

begin

read next input record

if record type = ‘T’ then

begin

{if object code is in character form, convert into internal representation}

move object code from record to location (CSADDR + specified address)

end {if ‘T’}

else if record type = ‘M’ then

begin

search ESTAB for modifying symbol name

if found then add or subtract symbol value at location (CSADDR + specified address)

else set error flag (undefined external symbol)

end {if ‘M’}

end {while != ‘E’}

if an address is specified {in End record} then set EXECADDR to (CSADDR + specified address)

add CSLTH to CSADDR

end {while not EOF}

Jump to location given by EXECDDR {to start execution of loaded program}

Pass2

(Perform actual loading, relocation, and linking)