CSCI341. Lecture 22, MIPS Programming: Directives, Linkers, Loaders, Memory

Transcription

1 CSCI341 Lecture 22, MIPS Programming: Directives, Linkers, Loaders, Memory

2 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)

3 REVIEW Six segments. object file header text segment data segment relocation information symbol table debugging info You should have a good understanding of what these segments are for.

4 ASSEMBLER DIRECTIVES.data.text.double Your new friend:.asciiz (and her brother,.ascii).globl

5 .ASCIIZ Store this string in consecutive bytes in memory, and null-terminate it.

6 .ASCII Store this string in consecutive bytes in memory, but don t null-terminate it.

7 EXAMPLE.asciiz liberty is like....byte 108, 105, 98, 101, 114, 116, 121, 0

8 .BYTE CRASHES YOUR.ASCIIZ PARTY!.byte Another directive you now know. Store n 8-bit values in successive bytes of memory.

9 .BYTE Store n 8-bit values in successive bytes of memory..byte b1, b2, b3, b4..., bn.byte 1, 8, 6, 7, 5, 3, 0, 9 addr val

10 EXAMPLE.asciiz liberty is like....byte 108, 105, 98, 101, 114, 116, 121, 0

11 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.

12 NULL TERMINATION A special character whose ascii code is 0.

13 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.

14 .ASCIIZ AND SYSCALL (printing a string on the console) Store a string in memory using.asciiz Print the string to the screen via syscall (we ll return to this after break)

15 LINKERS Separate compilation of files ( modules ) means we ve got to link them together before execution. (To resolve all unresolved labels and symbols)

16 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

17 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.

18 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.

19 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.

20 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

21 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!

22 LOADING (HELLO KERNEL!) The kernel is the central component of the operating system. It s the bridge between applications and the hardware layer. Handles system calls.

23 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

24 LOADING Lastly, if main returns, the startup routine terminates the program via an exit system call.

25 AFTER BREAK: Memory usage Stacks Procedure calling (B.6, 2.8)

26 HOMEWORK Reading 20 MIPS Assembly Language Programming Chapter 4 (online) Start reviewing for the midterm yeah midterms!