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Loader handoutAuthor: Nomair A. Naeem

This handout is intended to accompany the class lectures on theLoader and Relocator. The handout should NOT be considered asubstitute for the lecture. Dissemination of this handout (includ-ing posting on a website) is explicitly prohibited unless permission isobtained. Please report any errors to nanaeem@uwaterloo.ca.

1 Steps to manually create a MERL file in As-

sembly

The following steps create a MERL file (in ASSEMBLY) that can then be

assembled using an assembler that does not know of the MERL file format e.g.cs241.binasm.

.word A

........ .word B

Assembly

reloc1: .word A

........reloc2: .word B

..........

Add new labelsfor each use of

.word label

Add a labelto mark theend of code

reloc1: .word A

.........reloc2: .word B ............endCode:

Create TableEntries

reloc1: .word A

.........reloc2: .word B ............endCode:.word 1.word reloc1.word 1.word reloc2

The 1 here is a FORMATCODE for relocation

The next line gives thelocation to be relocated

We will see otherformat codes for Linking

Add a label tomark the

end of table

reloc1: .word A

.........reloc2: .word B ............endCode:.word 1.word reloc1.word 1.word reloc2endModule:

Add Header

0x10000002(i.e. beq $0, $0, 2).word endModule

.word endCode.........

reloc1: .word A .........

reloc2: .word B ............

endCode:.word 1.word reloc1.word 1.word reloc2endModule:The first instruction in the header results in a MERL file to act as a normal executable MIPS program.

If a relocator is not being used, the first instruction simply causes the program to jump the next two words in memory andhence the program can run without any relocation with starting memory address 0

If a relocator is used, the relocator knows where to find the information.

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The following example illustrates the MERL file in assembly, the memory ad-dresses for each line when loaded at starting address 0, and the actual 32-bit

binary for each line in the assembly file

; .merl VERSION OF RELOCATION EXAMPLE; MERL = MIPS executable relocatable linkable

; address machine l.beq $0, $0, 2 ; skip over header ; 0x00000000 0x10000002.word endmodule ; 0x00000004 0x0000003c.word endcode ; 0x00000008 0x0000002c

lis $3 ; 0x0000000c 0x00001814 .word 0xabc ; 0x00000010 0x00000abc lis $1 ; 0x00000014 0x00000814reloc1: .word A ; 0x00000018 0x00000024 jr $1 ; 0x0000001c 0x00200008 B:

jr $31 ; 0x00000020 0x03e00008 A:

beq $0, $0, B ; 0x00000024 0x1000fffereloc2: .word B ; 0x00000028 0x00000020endcode:

.word 1 ; relocate ; 0x0000002c 0x00000001

.word reloc1 ; location ; 0x00000030 0x00000018

.word 1 ; relocate ; 0x00000034 0x00000001

.word reloc2 ; location ; 0x00000038 0x00000028endmodule:

The last column above is the input to a RELOCATABLE LOADER i.e. a loaderwhich knows how to read the MERL format to relocate the code at any startingaddress . The relocating loader first loads the program at starting address as shown in the diagram on the following page:

(Note: in the diagram below the instruction beq is shown for clarity. Thislocation will in fact contain the 32-bit binary for the beq instruction from theheader. )

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--------------------------beq $0, $0, 2endModuleendCode--------------------------

CODE

--------------------------

TABLE

--------------------------

!

!+ 4!+ 8

!+ endCode

!+ endModule

OriginalAddress

048

endCode

endModule

RelocatedAddress

The relocator runs the following code to relocate each relocation entry:

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i + MEM[+8] The third word in the original code contained the addressof the endCode label.

Since the program is now loaded at , the 3rd wordis +8. Therefore we read the memory at this locationThis gives us the value of endCode in the original codeSince the program is loaded at we add to this AGAIN!!

end + MEM[+4] The 2nd word in memory contains the address of the end of So we load it through MEM[+4]The value at that address was based on addresses starting atso we add

while ( i < end){ This loop takes us through the footer table containingrelocation entries

if (MEM[i] == 1) { Remember that each entry is 2 words. The first word has valONE to indicate a relocation entry

MEM[

+ MEM[i+4]] +=

Since MEM[i] is the .word 1, MEM[i+4] is the address thatcontained a .word label.Since we started at , the .word label is actually at+ MEM[i+4]We READ in that value MEM[+ MEM[i+4]] and thisrepresents what .word label would have had we started at 0.Add to this, and STORE IT BACK

i +=8; Jump to next table entry}

elseERROR

}

TOOLS

Relocation Tool: cs241.merl

Input: merl file and relocation address

Output: NON-relocatable mips file (MERL HEADER AND FOOTERHAS BEEN REMOVEDready to load at given address

NOTE: mips.twoint, mips.address CAN take a second argument: an ad-dress at which to load a mips file

Relocatable Assembler: cs241.relasmA tool which knows what to write in the header and table to create a relocatablemerl file

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Semantic Rules for WLPP

CS 241

Syntactic Categories

E {expr, term, factor, lvalue} T {test} S{statement, statements} {int, int}

Type Derivation Rules

[Literals and identifiers]NUM :int NULL :int

hid, i decls

id:

[Parenthesized expressions] E :

(E) :

[Pointers] E :int

&E: int

E :int

E: int

E :int

new int [E] :int

[Addition] E1: int E 2: int

E1 + E2: int

E1: int E2: int

E1 + E2: int

E1: int E 2: int

E1 + E2: int

[Subtraction] E1: int E 2: int

E1 E2 : int

E1: int E2: int

E1 E2: int

E1: int E2: int

E1 E2: int

[Multiplication and division] E1: int E 2 : int

E1 E2: int

E1 : int E 2: int

E1/ E2 : int

E1: int E 2: int

E1% E2 : int

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Type Correctness Rules

[Comparisons] E1: E2:

well-typed(E1 == E2)

E1: E2:

well-typed(E1! = E2)

E1 : E2:

well-typed(E1 < E2)

E1: E2:

well-typed(E1 E2)

E1 : E2:

well-typed(E1 >= E2)

[Control flow] well-typed(T) well-typed(S)

well-typed(while ( T) { S})

well-typed(T) well-typed(S1) well-typed(S2well-typed(if ( T) { S1} else { S2})

[Deallocation] E : int

well-typed(delete [ ] E; )

[Printing] E: int

well-typed(printlnE; )

[Assignment] E1: E2:

well-typed(E1 = E2; )

[Sequencing]well-typed()

well-typed(S1) well-typed(S2)

well-typed(S1 S2)

[Declns]well-typed()

well-typed(dcls)well-typed(dclsint id= NUM; )

well-typed(dcls)well-typed(dclsint* id= NULL

[Procedure] decl2: int well-typed(decls) well-typed(S) E : int

well-typed(int wain(decl1, decl2) {decls Sreturn E ;})

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