CSCI341 Lecture 22, MIPS Programming: Directives, Linkers, Loaders, - - PowerPoint PPT Presentation

Lecture 22, MIPS Programming: Directives, Linkers, Loaders, Memory

CSCI341

REVIEW

• Assemblers understand special commands called “directives”
• Assemblers understand “macro” commands
• Assembly programs become object files
• Object files are structured in a specific way (six segments)

REVIEW

Six segments.

• bject file

header text segment data segment

relocation information

symbol table debugging info

You should have a good understanding of what these segments are for.

ASSEMBLER DIRECTIVES

• .data
• .text
• .double
• .globl

Your new friend: .asciiz (and her brother, .ascii)

.ASCIIZ

“Store this string in consecutive bytes in memory, and null-terminate it.”

.ASCII

“Store this string in consecutive bytes in memory, but don’t null-terminate it.”

EXAMPLE

.asciiz “liberty”

.byte 108, 105, 98, 101, 114, 116, 121, 0

is like...

.BYTE CRASHES YOUR .ASCIIZ PARTY!

.byte Another directive you now know. “Store n 8-bit values in successive bytes of memory.”

.BYTE

.byte 1, 8, 6, 7, 5, 3, 0, 9

“Store n 8-bit values in successive bytes of memory.”

.byte b1, b2, b3, b4 ..., bn

addr 1 2 3 4 5 6 7 val 1 8 6 7 5 3 9

EXAMPLE

.asciiz “liberty”

.byte 108, 105, 98, 101, 114, 116, 121, 0

is like...

EXAMPLE

.asciiz directives are often labeled. Why?

.data welcome: .asciiz “Welcome to the jungle!” .text # assembly code here...

“Store the string Welcome to the jungle! and remember it’s base address as welcome.”

NULL TERMINATION

A “special” character whose ascii code is 0.

WHY NULL-TERMINATION?

Common functions such as println or the OS syscall(8)will read your ascii data beginning with the base address until the null character is “found.”

.ASCIIZ AND SYSCALL

• Store a string in memory using .asciiz
• Print the string to the screen via syscall
• (we’ll return to this after break)

(printing a string on the console)

LINKERS

• Separate compilation of files (“modules”) means we’ve got to

“link” them together before execution.

• (To resolve all unresolved labels and symbols)

LINKERS

• Search program libraries to find library function calls in the

program

• Determine memory locations that each module’s instructions

will occupy, and relocate those instructions by adjusting absolute references.

• (Remember, “the shelf may move,” so instructions

referencing something on the shelf must change.)

• Resolve references among modules

LINKER MISSION #1

• Ensure the program has no undefined labels.
• Aside: remember all those compiler warnings about unused or

“unmatched” variable references?

• Those are symbols that were defined but never used.

LINKER MISSION #2

• If a program uses a library function, “extract” the function’s

code from the library and integrate it into the final executable.

• (Keep in mind library functions may call other library

functions)

• Continue this process until all external references are

resolved.

LINKER MISSION #3

• Determine the memory locations each “module” will occupy.
• Absolute address references must be relocated.
• Process is assisted by relocation information segment of

each module’s object file.

LINKER COMPLETE?

• Produces executable file.
• Typically has same segment-format as object file, except:
• There are no longer unresolved references
• No relocation information is stored

LOADING

• Executable programs first reside on disk (secondary storage)
• To “execute” a program means to:
• Load the executable file into memory
• Set the program counter to the base address of what was

just loaded

• Go!

LOADING (HELLO KERNEL!)

The “kernel” is the central component

• f the operating system.

It’s the “bridge” between applications and the hardware layer. Handles system calls.

LOADING

• OS reads the file’s header to determine size of text and data

segments

• Creates space in memory for the program (instructions, data

and a “workspace” or “stack”)

• Copies instructions and data into memory
• Copies arguments passed to the program onto the stack
• Initializes the machine registers (sets the stack pointer)
• Jumps to a startup routine that calls the main routine

LOADING

• Lastly, if main returns, the startup routine terminates the

program via an exit system call.

AFTER BREAK:

• Memory usage
• Stacks
• Procedure calling (B.6, 2.8)

• Reading 20
• MIPS Assembly Language Programming
• Chapter 4 (online)
• Start reviewing for the midterm

HOMEWORK

yeah midterms!