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08-assembly-64bit.html
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<!doctype html>
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<title>CS 2150: 08-x86 (64 bit) slide set</title>
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<div class="reveal">
<!-- Any section element inside of this container is displayed as a slide -->
<div class="slides">
<section data-markdown id="cover"><script type="text/template">
# CS 2150
### Program and Data Representation
<p class='titlep'> </p>
<div class="titlesmall"><p>
<a href="http://www.cs.virginia.edu/~mrf8t">Mark Floryan</a> ([email protected])<br>
<a href="http://www.cs.virginia.edu/~asb">Aaron Bloomfield</a> ([email protected])<br>
<a href="http://github.com/uva-cs/pdr">@github</a> | <a href="index.html">↑</a> | <a href="./08-assembly-64bit.html?print-pdf"><img class="print" width="20" src="../slides/images/print-icon.png" style="top:0px;vertical-align:middle"></a>
</p></div>
<p class='titlep'> </p>
## 64 bit x86 (assembly language)
</script></section>
<section>
<h2>CS 2150 Roadmap</h2>
<table class="wide">
<tr><td colspan="3"><p class="center">Data Representation</p></td><td></td><td colspan="3"><p class="center">Program Representation</p></td></tr>
<tr>
<td class="top"><small> <br> <br>string<br> <br> <br> <br>int x[3]<br> <br> <br> <br>char x<br> <br> <br> <br>0x9cd0f0ad<br> <br> <br> <br>01101011</small></td>
<!-- image adapted from http://openclipart.org/detail/3677/arrow-left-right-by-torfnase -->
<td><img class="noborder" src="images/red-double-arrow.png" height="500" alt="vertical red double arrow"></td>
<td class="top"> <br>Objects<br> <br>Arrays<br> <br>Primitive types<br> <br>Addresses<br> <br>bits</td>
<td> </td>
<td class="top"><small> <br> <br>Java code<br> <br> <br>C++ code<br> <br> <br>C code<br> <br> <br>x86 code<br> <br> <br>IBCM<br> <br> <br>hexadecimal</small></td>
<!-- image adapted from http://openclipart.org/detail/3677/arrow-left-right-by-torfnase -->
<td><img class="noborder" src="images/green-double-arrow.png" height="500" alt="vertical green double arrow"></td>
<td class="top"> <br>High-level language<br> <br>Low-level language<br> <br>Assembly language<br> <br>Machine code</td>
</tr>
</table>
</section>
<section data-markdown><script type="text/template">
# Contents
[Introduction to x86](#/introduction)
[x86 Instruction Set](#/x86insts)
[Calling Conventions](#/callingconv)
[Callee Rules](#/callee)
[Caller Rules](#/caller)
[Activation Records](#/actrecs)
[x86 Examples](#/examples)
</script></section>
<section>
<section id="introduction" data-markdown class="center"><script type="text/template">
# Introduction to x86
</script></section>
<section>
<h2>History of x86</h2>
<p><a href="https://en.wikipedia.org/wiki/File:C4004_%28Intel%29.jpg"><img alt="Intel 4004" src="images/08-x86/C4004.jpg"></a></p>
<table class="transparent"><tr><td>
<ul>
<li>1971: 4004, 4-bit words</li>
<li>1972: 8008, 8-bit words</li>
<li>1978: 8086, 16-bit words</li>
<li>1982: 80286</li>
<li>1985: 80386, 32-bit words</li>
<li>1989: 80486</li>
<li>1993: Pentium</li>
<li>1995: Pentium Pro</li>
</ul>
</td><td>
<ul>
<li>1997: Pentium II</li>
<li>1998: Pentium III</li>
<li>2000-2008: Pentium IV</li>
<li>2003: AMD64 Opteron</li>
<li>2004: Intel 64 bit chips</li>
<li>2005-2008: Pentium D</li>
<li>2006-2011: Core 2</li>
<li>2008-present: Core i3, i5, i7</li>
</ul>
</td></tr></table>
</section>
<section data-markdown><script type="text/template">
## AMD vs Intel
- AMD developed their standard in 2000
- First chip was available in 2003
- Intel had been working on their IA64 (Itanium) line
- There is a reason you have never heard of it
- They released their first chip, similar to the AMD64, in 2004
- But it had slight differences!
- The x86-64 standard is what compilers typically use, and is the intersection of the two instruction sets
- More than 95% of it is the same
</script></section>
<section>
<h2>IBCM vs. x86: Registers</h2>
<img src="images/08-x86/registers_x86_64bit.png" class="stretch" style="background-color:white;padding:10px;border-radius:10px">
</section>
<section data-markdown><script type="text/template">
## Funny register names
| 8-bit name | 16-bit name | 32-bit name | 64-bit name |
|--|--|--|--|
| a (al/ah) | ax | eax | rax |
| b (bl/bh) | bx | ebx | rbx |
| c (cl/ch) | cx | ecx | rcx |
| d (dl/dh) | dx | edx | rdx |
| - | - | esi | rsi |
| - | - | edi | rdi |
| - | sp | esp | rsp |
| - | bp | ebp | rbp |
| - | - | - | r8 - r15 |
</script></section>
<section data-markdown><script type="text/template">
## IBCM vs. x86: Fetch Execute Cycle (same)
```
while(power is on) {
IR := mem[PC]
PC := PC + 1 (word) // 64-bits in x86
execute instruction in IR
}
```
- PC = program counter
- IR = instruction register
</script></section>
<section>
<h2>Declaring Variables in x86</h2>
<table class="transparent"><tr><td>
<p>Directives</p>
<ul>
<li>byte: 1 byte (DB) declare byte</li>
<li>word: 2 bytes (DW)</li>
<li>double: 4 bytes (DD)</li>
<li>quadword: 8 bytes (DQ)</li>
</ul>
<p> </p>
<p><code>TIMES x DB 0</code> directive means create x bytes of value zero</p>
</td><td>
<pre><code>section .data
a DB 23
b DW ?
c DD 3000
d DQ -800
x DD 1, 2, 3
y TIMES 8 DB 0
str DB 'hello', 0
z TIMES 50 DD ?</code></pre>
</td></tr></table>
</section>
<section data-markdown><script type="text/template">
## mov command
- `mov <dest>, <src>`
- Where dest and src can be:
- A register
- A constant
- Variable name
- Pointer: [rbx]
- *You will often see movq (move quad word) or movl (move double word), etc.
- This is more precise as you are declaring how many bytes will be moved, but depends on the assembly syntax used (more on this later)
</script></section>
<section>
<h2>Addressing Memory</h2>
<table class="transparent"><tr><td class="top">
<ul>
<li>Up to <i>2 registers</i> and one <i>64-bit signed constant</i> can be added together to compute a memory address<br> </li>
<li>Furthermore, one register can be pre-multiplied by 2, 4, or 8<ul>
<li>word-align</li>
<li>double-align</li>
<li>quadword-align</li>
</ul></li></ul>
</td><td style="width:50px"></td><td class="top">
<pre><code>mov rax, rbx
mov rax, [rbx]
mov [var], rbx
mov rax, [r13 - 4]
mov [rsi + rax], cl
mov rdx, [rsi + 4*rbx]</code></pre>
<p> </p>
<p>Incorrect: (why?)</p>
<pre><code>mov rax, [r11 - rcx]
mov [rax + r5 + rdi], rbx
mov [4*rax + 2*rbx], rcx</code></pre>
</td></tr></table>
</section>
<section>
<table class="transparent"><tr><td class="top">
<table class="transparent">
<tr><td class="top" colspan="3"><h2>Example</h2></td></tr>
<tr><td class="top"><p class="center">Source code:</p><pre><code>mov rcx, rax
mov rdx, [rbx]
mov rsi, [rdx+rax+16]
mov [rsi], 45
mov [a], 15
lea rdi, [a]</code></pre></td>
<td style="width:50px"></td>
<td class="top">
<p class="center">Registers:</p>
<table class="transparent">
<tr><td>rax</td><td class="border" style="width:100px">100</td></tr>
<tr><td>rbx</td><td class="border" style="width:100px">108</td></tr>
<tr><td>rcx</td><td class="border" style="width:100px"><span class="fragment" data-fragment-index="1">100</span></td></tr>
<tr><td>rdx</td><td class="border" style="width:100px"><span class="fragment" data-fragment-index="2">8</span></td></tr>
<tr><td>rsi</td><td class="border" style="width:100px"><span class="fragment" data-fragment-index="3">200</span></td></tr>
<tr><td>rdi</td><td class="border" style="width:100px;border-bottom:medium solid;"><span class="fragment" data-fragment-index="6">300</span></td></tr>
<tr><td>r8</td><td class="border" style="width:100px;border-bottom:medium solid;"></td></tr>
<tr><td>...</td><td class="border" style="width:100px;border-bottom:medium solid;"></td></tr>
</table>
</td></tr></table>
</td><td style="width:50px"></td><td class="top">
<p class="center">Memory:</p>
<table class="transparent">
<tr><td>100</td><td class="border" style="width:100px"></td></tr>
<tr><td>108</td><td class="border" style="width:100px">8</td></tr>
<tr><td>116</td><td class="border" style="width:100px"></td></tr>
<tr><td>124</td><td class="border" style="width:100px">200</td></tr>
<tr><td>132</td><td class="border" style="width:100px"></td></tr>
<tr><td>...</td><td class="border" style="width:100px"></td></tr>
<tr><td>200</td><td class="border" style="width:100px"><span class="fragment" data-fragment-index="4">45</span></td></tr>
<tr><td>208</td><td class="border" style="width:100px"></td></tr>
<tr><td>...</td><td class="border" style="width:100px"></td></tr>
<tr><td>a: 300</td><td class="border" style="width:100px"><span class="fragment" data-fragment-index="5">15</span></td></tr>
<tr><td>308</td><td class="border" style="width:100px"></td></tr>
<tr><td>...</td><td class="border" style="width:100px;border-bottom:medium solid;"></td></tr>
</table>
</td></tr></table>
</section>
<section data-markdown><script type="text/template">
## `mov rax, [4*rsi-rdx]`
1. Valid
2. Invalid
3. Not sure
</script></section>
<section data-markdown><script type="text/template">
## `mov rax, [4*rsi+4]`
1. Valid
2. Invalid
3. Not sure
</script></section>
<section data-markdown><script type="text/template">
## `mov rax, [4*rsi+rdx+8]`
1. Valid
2. Invalid
3. Not sure
</script></section>
<section data-markdown><script type="text/template">
## `mov rax, [rsi+4*rdx]`
1. Valid
2. Invalid
3. Not sure
</script></section>
<section data-markdown><script type="text/template">
## `mov rax+8, [rsi]`
1. Valid
2. Invalid
3. Not sure
</script></section>
<section data-markdown><script type="text/template">
## Memory addressing restrictions
- The destination cannot be a constant (duh!)
- You cannot access memory twice in one instruction
- As the CPU does not have enough time to do so at that clock speed
- So the following instructions are invalid:
```
mov [rax], [var]
mov [rax+8], [rbx]
mov 20, [rax]
```
</script></section>
</section>
<section>
<section data-markdown id="x86insts" class="center"><script type="text/template">
# x86 Instruction Set
</script></section>
<section data-markdown><script type="text/template">
## x86 Instruction Set
- Data movement instructions
- Arithmetic instructions
- Logical instructions
- Control instructions
</script></section>
<section data-markdown><script type="text/template">
## Data Movement Instructions
- `mov` (or `movq` if you are moving 8 bytes)
- We've seen this in detail already
- `push`
- First decrements register RSP (stack pointer) by 8 (stack grows ***down***)
- push (mov) operand onto stack (8 bytes)
- sometimes you will see this as pushq (push quad word onto stack)
```
push rax
push [var]
```
</script></section>
<section data-markdown><script type="text/template">
## Data Movement Instructions
- `pop`
- Pop top element of stack to memory or register, then increment stack pointer (RSP) by 8
- Value is written to the parameter
```
pop rax
pop [var]
```
- `lea`
- Load effective address
- Place address of second parameter into the first parameter
```
lea rax, [var]
lea rdi, [rbx+4*rsi]
```
</script></section>
<section data-markdown><script type="text/template">
## Arithmetic Instructions
- `add`, `sub`
```
add <reg>, <reg>
add <reg>, <mem>
add <mem>, <reg>
add <reg>, <constant>
add <mem>, <constant>
```
- Adds (or subtracts), storing result in first operand
- Similar restrictions as with data movement instructions:
- Destination cannot be a constant
- Memory cannot be accessed twice
</script></section>
<section data-markdown><script type="text/template">
## Arithmetic Instructions
- `inc`, `dec` (increment and decrement by one)
```
inc <reg>
inc <mem>
```
- Specific examples:
```
dec rax
inc [var]
```
- `imul`
```
imul <reg64>, <reg64> (or <mem>)
imul <reg64>, <reg64> (or <mem>), <con>
```
- `idiv`
- Divide 128-bit integer in RDX:RAX by operand
```
idiv rbx
```
</script></section>
<section data-markdown><script type="text/template">
## Logical Instructions
- `and`, `or`, `xor`
```
and <reg>, <reg>
and <reg>, <mem>
and <mem>, <reg>
```
- Specific examples:
```
and rax, 0fH
xor rcx, rcx
```
</script></section>
<section data-markdown><script type="text/template">
## Control Instructions, part 1
- `jmp <label>`
- go to instruction address specified by label
- `cmp`
- This must be done prior to an *conditional* jump
- cmp operand1, operand2
- Operand1 can be a register or memory (variable)
- Operand2 can be a register, memory (variable), or a constant
- Recall that you can't access memory twice!
- Sets the *machine status word*
</script></section>
<section data-markdown><script type="text/template">
## Control Instructions, part 2
- Conditional jumps: j*condition*
- Uses the machine status word, which was set via `cmp`
- Which holds info about the results of the last instruction
- There are many 'condition's to determine whether to jump
- Example: `je <label>`
- Jump when condition code equal is set
- Others: `jne`, `jz`, `jg`, `jge`, `jl`, `jle`, `js`, etc.
</script></section>
<section data-markdown><script type="text/template">
## Control Instructions, part 3
- `call <label>`
- Subroutine call
- Pushes address of the *next* instruction onto the stack, then unconditionally jumps to the label
- `ret`
- Subroutine return
- Pops the return address from the stack, then jumps to that address
</script></section>
<section>
<h2>A code block in both C/C++ and Assembly</h2>
<table class="transparent">
<tr><td><p>C/C++ code:</p><pre><code>int n = 5;
int i = 1;
int sum = 0;
while (i <= n) {
sum += i;
i++;
}</code></pre></td>
<td style="width:50px"></td>
<td><p>Assembly code:</p><pre><code>section .data
n DQ 5
i DQ 1
sum DQ 0
section .text
loop: mov rcx, [i]
cmp rcx, [n]
jg endOfLoop
add [sum], rcx
inc qword [i]
jmp loop
endOfLoop:
</code></pre></td></tr></table>
</section>
</section>
<section>
<section id="callingconv" data-markdown class="center"><script type="text/template">
# Calling Conventions
</script></section>
<section data-markdown><script type="text/template">
## Calling of a subroutine
```
int max(int x, int y) {
int theMax = (x > y) ? x : y;
return theMax;
}
int main() {
int a = 5, b = 6;
int maxVal = max(a,b);
cout << "Max value: " << maxVal << endl;
return 0;
}
```
</script></section>
<section data-markdown><script type="text/template">
## Calling Conventions
- What is a calling convention?
- A set of rules/expectations between functions
- How (and where) are *parameters* passed?
- Which *registers* does the calling function expect to be preserved?
- Where should *local variables* be stored?
- How/where should results be *returned* from functions?
- Why?
- Separate programmers can:
- Share code more easily
- Develop libraries
</script></section>
<section data-markdown><script type="text/template">
## C Calling Convention
- Why C's calling convention?
- It's important
- Is used with both C and C++ code
- Can enable calling C library functions from assembly code
- Or other languages, too
</script></section>
<section data-markdown><script type="text/template">
## C Calling Convention
- Uses hardware stack (memory)
- Stack *grows down*, toward the lower memory addresses
- x86 instructions used for calling convention
- `pop`
- `push`
- `call`
- `ret`
- Using a stack for calling convention is implemented on most processors. Why?
- Recursion
</script></section>
<section data-markdown><script type="text/template">
## C Calling Convention Overview
- Answers to questions
- Parameters: passed in registers
- If more than 6, then params 7-*n* placed on stack
- If passing a large object, then placed on stack (doesn't fit in 64-bit register)
- Registers: saved on the stack
- Local variables: placed in memory on the stack
- Or in registers if room available
- Return value: rax register
</script></section>
<section data-markdown><script type="text/template">
## Calling Convention Overview
- Two sets of rules
- Caller: the function which calls another function
- Callee: the function which is called by another function
</script></section>
<section data-markdown><script type="text/template">
## Caller vs. Callee
```
void foo() {
// some function code ...
}
int main() {
foo();
return 0;
}
```
- `main()` is the *caller*
- `foo()` is the *callee*
</script></section>
<section data-markdown><script type="text/template">
## Register usage
- One register is used for the return value: rax
- Six registers are used for parameter passing:
- rdi, rsi, rdx, rcx, r8, r9
- Two registers may be modified by the callee: r10 and r11
- If the caller wants to keep those values, they need to be saved by pushing them onto the stack
- Actually, these two have specific uses depending on the language, so are often not used for this reason.
- Six registers may not be modified by a subroutine callee:
- rbx, rbp, r12-r15
- If it wants to use them, the subroutine must back them up (onto the stack) and restore them later
- rsp should almost never be modified directly, as it points to the top of the stack
</script></section>
<section data-markdown><script type="text/template">
## Varying number of parameters
- Three ways to have a variable number of parameters:
- Method overloading
- `Foo::bar(int)` and `Foo::bar(int, float)`
- Default parameters
- `Foo::bar (int x = 3)`
- Variable arguments
- As seen next...
</script></section>
<section data-markdown><script type="text/template">
## Variable number of arguments in C/C++
This code adapted from [here](http://www.cprogramming.com/tutorial/lesson17.html)
```
#include <cstdarg>
#include <iostream>
using namespace std;
double average (int num, ...) {
va_list arguments;
double sum = 0;
va_start (arguments, num);
for ( int x = 0; x < num; x++ )
sum += va_arg (arguments, double);
va_end (arguments);
return sum / num;
}
int main() {
cout << average(3, 12.2, 22.3, 4.5) << endl;
cout << average(5, 3.3, 2.2, 1.1, 5.5, 3.3) << endl;
}
```
</script></section>
<section data-markdown><script type="text/template">
## Dissection of the average() function
- Create a data structure to hold the list of arguments:
```
va_list arguments;
```
- Initialize the arguments to store all values after num:
```
va_start ( arguments, num );
```
- Loop until all numbers are added:
```
for ( int x = 0; x < num; x++ )
```
- Adds the next value in argument list to sum:
```
sum += va_arg ( arguments, double );
```
- Clean up the list:
```
va_end ( arguments );
```
- Return the average:
```
return sum / num;
```
</script></section>
<section data-markdown><script type="text/template">
## Example: Output in C
The C equivalent of cout is a function called `printf`
```
printf ("A %s, a %s, a %s: %s!\n", "man",
"plan", "canal", "Panama");
printf ("A percent sign: %%\n");
printf ("An int: %d\n", i);
printf ("A float with 2 decimal digits: %.2f\n",
float_value);
```
Output:
```
A man, a plan, a canal: Panama!
A percent sign: %
An int: 3
A float with 2 decimal digits: 3.14
```
</script></section>
</section>
<section>
<section id="caller" data-markdown class="center"><script type="text/template">
# Caller Rules
</script></section>
<section data-markdown><script type="text/template">
## Caller Summary
- Prologue
- Tasks to take care of BEFORE calling a subroutine
- Call the subroutine with the `call` opcode
- Epilogue
- Tasks to complete AFTER subroutine call returns
- (It is not really called this, but I use this to parallel the equivalent components in the callee convention)
</script></section>
<section data-markdown><script type="text/template">
## Callee Summary
- Prologue
- Tasks to perform BEFORE executing the function body
- Function body
- Epilogue
- Tasks to perform AFTER executing function body, but BEFORE leaving function
</script></section>
<section data-markdown><script type="text/template">
## Caller Rules/Responsibilities
- *Before* calling the function (the prologue)
- Save registers that might be needed after the call (r10, r11, or param registers if applicable)
- Place parameters in registers / on stack
- rdi, rsi, rdx, rcx, r8, r9
- Then push extra params onto stack
- *Call* the function
- `call` instruction places return address on stack
- *After* the called function returns (the epilogue)
- Remove parameters from stack (if applicable)
- Restore saved registers (if applicable)
</script></section>
<section data-markdown><script type="text/template">
## Caller Rules ("Prologue")
1. Caller-saved registers
- Registers which the caller must save (push onto the stack) ONLY if it wants the values preserved.
- r10, r11, and registers used for parameters if need value saved
2. Parameters
- First six are passed in through registers (see previous slide)
- params 7-n pushed in *reverse order* (last parameter first) onto stack
3. Call the subroutine
- Use the `call` instruction
- pushes the return address onto the stack and branches to the subroutine
</script></section>
<section data-markdown><script type="text/template">
## Caller Rules ("Epilogue")
1. Remove parameters
- Parameters pushed onto stack must be removed
- Restore stack to the state before the call
- What is done with the parameters?
2. Return value
- If any, held in rax
3. Restore caller-saved registers
- `pop` them off the stack (Caller can assume no other registers were modified)
</script></section>
<section data-markdown><script type="text/template">
## Caller Rules Example
We'll see this code in the following slides:
```
long myFunc(long a, long b, long c) {
long result;
// some code
return result;
}
int main() {
long x = 1, z = 3;
long retVal = myFunc(x, 123, z);
//...
return 0;
}
```
</script></section>
<section data-markdown><script type="text/template">
## Caller Rules Example
```
; Want to call a function "myFunc" that takes three
; integer parameters. First parameter is in rax.
; Second parameter is the constant 123. Third
; parameter is in memory location "var"
push rdi ; rdi will be a param, so saving it
; long retVal = myFunc(x, 123, z);
mov rdi, rax ; put first param in rdi
mov rsi, 123 ; put second param in rsi
mov rdx, [var] ; put third param in rdx
call myFunc ; call the function
pop rdi ; restore saved rdi value
; return value of myFunc is now available in rax
; (if there is any return value)
```
</script></section>
<section>
<h2>Stack Memory Visualization for myFunc</h2>
<p>This is just before the <code>call</code> opcode is invoked.</p>
<hr>
<table class="transparent">
<tr><td>↑</td><td style="width:50px"></td><td class="border" style="width:300px">value of rdi</td><td style="width:25px"></td><td>← rsp</td></tr>
<tr><td>To higher addresses</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>(to 0xffffffff)</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>To lower addresses</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>(to 0x00000000)</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>↓</td><td style="width:50px"></td><td class="border" style="width:300px;border-bottom:medium solid;"> </td><td style="width:25px"></td><td> </td></tr>
</table>
</section>
<section data-transition="fade" data-transition-speed="fast">
<h2>Stack Memory Visualization for myFunc</h2>
<p>This is just <i><b>after</b></i> the <code>call</code> opcode is invoked.</p>
<hr>
<table class="transparent">
<tr><td>↑</td><td style="width:50px"></td><td class="border" style="width:300px">value of rdi</td><td style="width:25px"></td><td> </td></tr>
<tr><td>To higher addresses</td><td style="width:50px"></td><td class="border" style="width:300px">return address</td><td style="width:25px"></td><td>← rsp</td></tr>
<tr><td>(to 0xffffffff)</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>To lower addresses</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>(to 0x00000000)</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>↓</td><td style="width:50px"></td><td class="border" style="width:300px;border-bottom:medium solid;"> </td><td style="width:25px"></td><td> </td></tr>
</table>
</section>
</section>
<section>
<section id="callee" data-markdown class="center"><script type="text/template">
# Callee Rules
</script></section>
<section data-markdown><script type="text/template">
## Callee Summary (again)
- Prologue
- Tasks to perform BEFORE executing the function body
- Function body
- Epilogue
- Tasks to perform AFTER executing function body, but BEFORE leaving function
</script></section>
<section data-markdown><script type="text/template">
## Caller Rules Example
We will see this code in the following slides:
```
long myFunc(long a, long b, long c) {
long result;
// some code
return result;
}
int main() {
long x = 1, z = 3;
long retVal = myFunc(x, 123, z);
//...
return 0;
}
```
</script></section>
<section data-markdown><script type="text/template">
## Callee Rules (Prologue)
Before the body of the function:
1. Allocate local variables
- Make space on stack (decrement stack pointer)
```
sub rsp, 8
```
That is it. NOTE: caller is also allowed to use valid registers for local variables, so sometimes you will see that instead.
</script></section>
<section data-markdown id="pushrbp"><script type="text/template">
## PUBLIC SERVICE ANNOUNCEMENT!
- Note that you might see the following:
```
push rbp ; at the start of the callee
mov rbp, rsp
...
pop rbp ; just before the ending 'ret'
```
- This is a 32-bit x86 convention that is no longer officially used, but is sometimes seen anyway.
- To omit it, add the `-fomit-frame-pointer` flag to the compilation line
</script></section>
<section>
<h2>Callee Rules (Prologue)</h2>
<ol start="3">
<li>Save callee-save registers<ul>
<li>rbx, rbp, r12-r15</li>
<li>only need to do this if callee intends to use them, otherwise, no need to save their contents</li>
</ul></li></ol>
<p> </p>
<p>THEN, perform body of the function</p>
</section>
<section data-markdown><script type="text/template">
## Callee Rules (Epilogue)
1. Return value saved to rax
2. Restore callee-saved registers
- `pop` from stack (in reverse order from which pushed)
3. Deallocate local variables
```
add rsp, 8 //constant here depends on size of locals
```
5. Return
```
ret
```
</script></section>
<section data-markdown><script type="text/template">
## Callee Rules Example
With a bit more code in the `myFunc()` body (NOTE the use of long here because they are 64-bit):
```
long myFunc(long a, long b, long c) {
long result;
result = c;
result += b;
return result;
}
int main() {
long x = 1, z = 3;
long retVal = myFunc(x, 123, z);
//...
return 0;
}
```
</script></section>
<section data-markdown><script type="text/template">
## Callee Rules Example (1)
```
section .text
myFunc:
; prologue
sub rsp, 8 ; room for a 64-bit local var (result)
push rbx ; save callee-save registers
push rbp ; both will be used by myFunc
```
</script></section>
<section data-markdown><script type="text/template">
## Callee Rules Example (2)
```
; subroutine body
mov rax, rdi ; param 1 to rax
mov rbp, rsi ; param 2 to rbp
mov rbx, rdx ; param 3 to rbx
mov [rsp+16], rbx ; put rbx into local var
add [rsp+16], rbp ; add rbp into local var
mov rax, [rsp+16] ; mov contents of local var to rax
; (return value/final result)
```
</script></section>
<section data-markdown><script type="text/template">
## Callee Rules Example (3)
```
; subroutine epilogue
pop rbp ; recover callee save registers
pop rbx ; REVERSE of when pushed
add rsp, 8 ; deallocate local var(s)
ret ; pop top value from stack, jump there
```
</script></section>
<section>
<h2>Stack Memory Visualization for myFunc</h2>
<p>This is just <i>after</i> the caller invokes the <code>call</code> opcode.</p>
<hr>
<table class="transparent">
<tr><td>↑</td><td style="width:50px"></td><td class="border" style="width:300px">value of rdi</td><td style="width:25px"></td><td> </td></tr>
<tr><td>To higher addresses</td><td style="width:50px"></td><td class="border" style="width:300px">return address</td><td style="width:25px"></td><td>← rsp</td></tr>
<tr><td>(to 0xffffffff)</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>To lower addresses</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>(to 0x00000000)</td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td>↓</td><td style="width:50px"></td><td class="border" style="width:300px;border-bottom:medium solid;"> </td><td style="width:25px"></td><td> </td></tr>
</table>
</section>
<section data-transition="fade" data-transition-speed="fast">
<h2>Stack Memory Visualization for myFunc</h2>
<p>This is just after the callee invokes the <code>sub rsp, 8</code> opcode.</p>
<hr>
<table class="transparent">
<tr><td>↑</td><td style="width:50px"></td><td class="border" style="width:300px">value of rdi</td><td style="width:25px"></td><td> </td></tr>
<tr><td>To higher addresses</td><td style="width:50px"></td><td class="border" style="width:300px">return address</td><td style="width:25px"></td><td> </td></tr>
<tr><td>(to 0xffffffff)</td><td style="width:50px"></td><td class="border" style="width:300px">local var (result)</td><td style="width:25px"></td><td>← rsp</td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>
<tr><td></td><td style="width:50px"></td><td class="border" style="width:300px"> </td><td style="width:25px"></td><td> </td></tr>