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.

object file header

text segment

data segment

relocation information

symbol table

debugging info

You should have a good understanding ofwhat 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 0 1 2 3 4 5 6 7

val 1 8 6 7 5 3 0 9

EXAMPLE

.asciiz “liberty”

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

is like...

EXAMPLE

.asciiz directives are often labeled. Why?

.datawelcome: .asciiz “Welcome to the jungle!”

.text # assembly code here...

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

NULL TERMINATIONA “special” character whose ascii code is 0.

WHY NULL-TERMINATION?

Common functions such as println or theOS 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 componentof the operating system.

It’s the “bridge” between applications andthe 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!