1. 8

2. The 20 MIPS Instructions Covered So Farop 15 8 10 12 13 14 35 43 2 1 4 5 fn 32 34 42 36 37 38 39 8
Instruction
Usage
Load upper immediate
lui rt,imm
Add 
add rd,rs,rt
Subtract
sub rd,rs,rt
Set less than
slt rd,rs,rt
Add immediate 
addi rt,rs,imm
Set less than immediate
slti rd,rs,imm
AND
and rd,rs,rt OR
or rd,rs,rt
XOR
xor rd,rs,rt
NOR
nor rd,rs,rt
AND immediate
andi rt,rs,imm
OR immediate
ori rt,rs,imm
XOR immediate
xori rt,rs,imm
Load word
lw rt,imm(rs)
Store word
sw rt,imm(rs)
Jump 
j L
Jump register
jr rs
Branch less than 0
bltz rs,L
Branch equal
beq rs,rt,L
Branch not equal 
bne rs,rt,L
Copy
Arithmetic
Logic
Memory access
Control transfer

3. Interpretation vs. Translation
How do we run a program written in a source language?
Interpreter: Directly executes a program in the source language. Examples?
Translator: Converts a program from the source language to an equivalent language in another language

4. Interpretation vs. Translation
Generally easier to write interpreter
Interpreter closer to high-level, so can give better error messages (e.g., SPIM)
Interpreter slower (10x?) but code is smaller (1.5x to 2x?)
Interpreter provides instruction set independence: run on any machine
Assuming interpreter written in a portable format

5. Interpretation vs. Translation
Translated/compiled code almost always more efficient and therefore higher performance
Important for many applications, particularly operating systems
Translation/compilation helps “hide” the program “source” from the users
One model for creating value in the marketplace (e.g., Microsoft keeps all their source code secret)
Alternative model, “open source”, creates value by publishing the source code and fostering a community of developers (e.g., Linux)

6. Steps to Starting a Program (translation)
C program: foo.c
Compiler
Assembly program: foo.s
Assembler
Object(mach lang module): foo.o
Linker
lib.o
Executable(mach lang pgm): a.out
Loader
Memory

7. Compiler Input: High-Level Language Code (e.g., C, Java such as foo.c)
Output: Assembly Language Code (e.g., foo.s for MIPS)
Note: Output may contain pseudoinstructions
Pseudoinstructions: instructions that assembler understands but not in machine. For example:
mov $s1,$s2  or $s1,$s2,$zero

8. MIPS Pseudo-instructions
Usage
Move
move regd,regs
Load address 
la regd,address
Load immediate
li regd,anyimm
Absolute value
abs regd,regs
Negate
neg regd,regs
Multiply (into register)
mul regd,reg1,reg2
Divide (into register)
div regd,reg1,reg2
Remainder
rem regd,reg1,reg2
Set greater than
sgt regd,reg1,reg2
Set less or equal
sle regd,reg1,reg2
Set greater or equal
sge regd,reg1,reg2
Rotate left
rol regd,reg1,reg2
Rotate right
ror regd,reg1,reg2
NOT
not reg
Load doubleword
ld regd,address
Store doubleword
sd regd,address
Branch less than
blt reg1,reg2,L
Branch greater than
bgt reg1,reg2,L
Branch less or equal
ble reg1,reg2,L
Branch greater or equal
bge reg1,reg2,L
Copy
Arithmetic
Shift
Logic
Memory access
Control transfer

9. Where Are We Now? C program: foo.c Compiler Assembly program: foo.s
Compiler writing course
Compiler
Assembly program: foo.s
Assembler
Object(mach lang module): foo.o
Linker
lib.o
Executable(mach lang pgm): a.out
Loader
Memory

10. Assembler Input: Assembly Language Code (e.g., foo.s for MIPS)
Output: Object Code, information tables (e.g., foo.o for MIPS)
Reads and Uses Directives
Replace Pseudoinstructions
Allow programmers to associate arbitrary names (labels or symbols) with memory locations
Produce Machine Language
Creates Object File

11. Assembler Directives (p. A-51 to A-53)
Give directions to assembler, but do not produce machine instructions
.text: Subsequent items put in user text segment (machine code)
.data: Subsequent items put in user data segment (binary rep of data in source file)
.globl sym: declares sym global and can be referenced from other files
.asciiz str: Store the string str in memory and null-terminate it
.word w1…wn: Store the n 32-bit quantities in successive memory words

12. Pseudoinstructions
Example of one-to-one pseudoinstruction: The following
not $s # complement ($s0)
is converted to the real instruction:
nor $s0,$s0,$zero # complement ($s0)
Example of one-to-several pseudoinstruction: The following
abs $t0,$s0 # put |($s0)| into $t0
is converted to the sequence of real instructions:
add $t0,$s0,$zero # copy x into $t0
slt $at,$t0,$zero # is x negative?
beq $at,$zero,+4 # if not, skip next instr
sub $t0,$zero,$s0 # the result is 0 – x

13. Pseudoinstruction Replacement
Assembler treats convenient variations of machine language instructions as if real instructions Pseudo: Real:
subu $sp,$sp,32 addiu $sp,$sp,-32
sd $a0, 32($sp) sw $a0, 32($sp) sw $a1, 36($sp)
mul $t7,$t6,$t5 mul $t6,$t5 mflo $t7
addu $t0,$t6,1 addiu $t0,$t6,1
ble $t0,100,loop slti $at,$t0,101 bne $at,$0,loop
la $a0, str lui $at,l.str ori $a0,$at,r.str

14. Example: C  Asm  Obj  Exe  Run
prog.c
#include <stdio.h>
int main (int argc, char *argv[]) {
int i, sum = 0;
for (i = 0; i <= 100; i++) sum = sum + i * i;
printf ("The sum from is %d\n", sum); }
printf lives in libc

15. Compilation: MIPS addu $t0, $t6, 1 .text sw $t9, 8($sp)
ble $t0,100, loop
la $a0, str
lw $a1, 8($sp)
jal printf
move $v0, $0
lw $ra, 4($sp)
addiu $sp,$sp,16
jr $ra
.data
.align 0
str:
.asciiz "The sum from is %d\n"
.text
.align 2
.globl main
main:
subu $sp,$sp,16
sw $ra,4($sp)
sd $a0, 16($sp)
sw $0, 8($sp)
sw $0, 12($sp)
loop:
lw $t6, 12($sp)
mul $t7, $t6,$t6
lw $t8, 8($sp)
addu $t9,$t8,$t7
sw $t9, 8($sp)
Where are 7 pseudo- instructions?

16. Compilation: MIPS addu $t0, $t6, 1 .text
sw $t0, 12($sp)
ble $t0,100, loop
la $a0, str
lw $a1, 8($sp)
jal printf
move $v0, $0
lw $ra, 4($sp)
addiu $sp,$sp,16
jr $ra
.data
.align 0
str:
.asciiz "The sum from is %d\n"
.text
.align 2
.globl main
main:
subu $sp,$sp,16
sw $ra, 4($sp)
sd $a0, 16($sp)
sw $0, 8($sp)
sw $0, 12($sp)
loop:
lw $t6, 12($sp)
mul $t7, $t6,$t6
lw $t8, 8($sp)
addu $t9,$t8,$t7
sw $t9, 8($sp)
7 pseudo-instructions underlined

17. Assembly step 1 Remove pseudoinstructions, assign addresses
00 addiu $29,$29,-16
04 sw $31,4($29)
08 sw $4, 16($29)
0c sw $5, 20($29)
10 sw $0, 8($29)
14 sw $0, 12($29)
18 lw $14, 12($29)
1c multu $14, $14
20 mflo $15
24 lw $24, 8($29)
28 addu $25,$24,$15
2c sw $25, 8($29)
30 addiu $8,$14, 1
34 sw $8,12($29)
38 slti $1,$8, 101
3c bne $1,$0, loop
40 lui $4, l.str
44 ori $4,$4,r.str
48 lw $5,8($29)
4c jal printf
50 add $2, $0, $0
54 lw $31,4($29)
58 addiu $29,$29,16
5c jr $31

18. Producing Machine Language
Simple Case
Arithmetic, Logical, Shifts, and so on
All necessary info is within the instruction already
What about Branches?
PC-Relative
So once pseudo-instructions are replaced by real ones, we know by how many instructions to branch
So these can be handled

19. Producing Machine Language
“Forward Reference” problem
Branch instructions can refer to labels that are “forward” in the program:
or $v0,$0,$ L1: slt $t0,$0,$a beq $t0,$0,L addi $a1,$a1, j L L2: add $t1,$a0,$a1
Solved by taking 2 passes over the program
First pass remembers position of labels
Second pass uses label positions to generate code

20. Producing Machine Language
What about jumps (j and jal)?
Jumps require absolute address
So, forward or not, still can’t generate machine instruction without knowing the position of instructions in memory
What about references to data?
la gets broken up into lui and ori
These will require the full 32-bit address of the data
These can’t be determined yet, so we create two tables…

21. Symbol Table
List of “items” in this file that may be used by other files
What are they?
Labels: function calling
Data: anything in the global part of the .data section; variables which may be accessed across files

22. Relocation Table List of “items” for which this file needs the address
What are they?
Any label jumped to: j or jal
internal
external (including lib files)
Any data label reference
such as the la instruction

23. Assembly step 2 Create relocation table and symbol table Symbol Table
Label Address (in module) Type
main: 0x global text
loop: 0x local text
str: 0x local data
Relocation Information
Address Instr. Type Dependency
0x lui l.str 0x ori r.str 0x c jal printf

24. Assembly step 3 Resolve local PC-relative labels 30 addiu $8,$14, 1
04 sw $31,4($29)
08 sw $4, 16($29)
0c sw $5, 20($29)
10 sw $0, 8($29)
14 sw $0, 12($29)
18 lw $14, 12($29)
1c multu $14, $14
20 mflo $15
24 lw $24, 8($29)
28 addu $25,$24,$15
2c sw $25, 8($29)
30 addiu $8,$14, 1
34 sw $8,12($29)
38 slti $1,$8, 101
3c bne $1,$0, -10
40 lui $4, l.str
44 ori $4,$4,r.str
48 lw $5,8($29)
4c jal printf
50 add $2, $0, $0
54 lw $31,4($29)
58 addiu $29,$29,16
5c jr $31

25. Assembly step 4 Generate object (.o) file
Output binary representation for
text segment (instructions)
data segment (data)
symbol and relocation tables
Using dummy “placeholders” or “guesses” for unresolved absolute and external references

28. Object File Format
object file header: size and position of the other pieces of the object file
text segment: the machine code
data segment: binary representation of the data in the source file
relocation information: identifies lines of code that need to be “handled”
symbol table: list of this file’s labels and data that can be referenced
debugging information
A standard format is ELF (except MS)

29. Where Are We Now? C program: foo.c Compiler Assembly program: foo.s
Assembler
Object(mach lang module): foo.o
Linker
lib.o
Executable(mach lang pgm): a.out
Loader
Memory

30. Linker Input: Object Code files (e.g., foo.o,libc.o for MIPS)
Output: Executable Code (e.g., a.out for MIPS)
Combines several object (.o) files into a single executable (“linking”)
Enable Separate Compilation of files
Changes to one file do not require recompilation of whole program
Windows NT source is > 40 M lines of code!
Old name “Link Editor” from editing the “links” in jump and link instructions

31. Morgan Kaufmann Publishers
April 14, 2017
FIGURE B.3.1 The linker searches a collection of object fi les and program libraries to find nonlocal routines used in a program, combines them into a single executable file, and resolves references between routines in different files. Copyright © 2009 Elsevier, Inc. All rights reserved.
Chapter 1 — Computer Abstractions and Technology

32. Linker .o file 1 text 1 a.out data 1 Relocated text 1 info 1
Relocated data 1
.o file 2
text 2
Relocated data 2
data 2
info 2

33. Linker
Step 1: Take text segment from each .o file and put them together
Step 2: Take data segment from each .o file, put them together, and concatenate this onto end of text segments
Step 3: Resolve References
Go through Relocation Table and handle each entry
That is, fill in all absolute addresses

34. Resolving References
Linker assumes first word of first text segment is at address 0x
(More on this later when we study “virtual memory”)
Linker knows:
length of each text and data segment
ordering of text and data segments
Linker calculates:
absolute address of each label to be jumped to (internal or external) and each piece of data being referenced

35. Resolving References To resolve references:
Based on list in each relocation table search for reference (label) in all “user” symbol tables
if not found, search library files (for example, for printf)
once absolute address is determined, fill in the machine code appropriately
Output of linker: executable file containing text and data (plus header)

36. The University of Adelaide, School of Computer Science
14 April 2017
FIGURE 2.13 The MIPS memory allocation for program and data. These addresses are only a software convention, and not part of the MIPS architecture. The stack pointer is initialized to 7fff fffchex and grows down toward the data segment. At the other end, the program code (“text”) starts at hex. The static data starts at hex. Dynamic data, allocated by malloc in C and by new in Java, is next. It grows up toward the stack in an area called the heap. The global pointer, $gp, is set to an address to make it easy to access data. It is initialized to hex so that it can access from hex to 1000 ffffhex using the positive and negative 16-bit offsets from $gp. This information is also found in Column 4 of the MIPS Reference Data Card at the front of this book. Copyright © 2009 Elsevier, Inc. All rights reserved.
Chapter 2 — Instructions: Language of the Computer

39. Problem
Link the object files above to form the executable file header. Assume that Procedure A has a text size of 0x140 and data size of 0x40 and Procedure B has a text size of 0x300 and data size of 0x50.

40. Text Size
????
Data Size
?????
Text
Address
Instruction
???????
lbu $a0, ????($gp)
?????????
jal ????? …
??????????
sw $a1, ????($gp)
jal ??????????
Data
???????????
(X)
(Y)

42. Static vs. Dynamically linked libraries
What we’ve described is the traditional way: “statically- linked” approach
The library is now part of the executable, so if the library updates we don’t get the fix (have to recompile if we have source)
It includes the entire library even if not all of it will be used
Executable is self-contained
An alternative is dynamically linked libraries (DLL), common on Windows & UNIX platforms
1st run overhead for dynamic linker-loader
Having executable isn’t enough anymore!

43. Where Are We Now? C program: foo.c Compiler Assembly program: foo.s
Assembler
Object(mach lang module): foo.o
Linker
lib.o
Executable(mach lang pgm): a.out
Loader
Memory

44. Loader Input: Executable Code (e.g., a.out for MIPS)
Output: (program is run)
Executable files are stored on disk
When one is run, loader’s job is to load it into memory and start it running
In reality, loader is the operating system (OS)
loading is one of the OS tasks