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amd64

rax return value, caller saved.
rbx base register (used for mem basepointer)
rcx counter register
rdx data register, multiplication and division operations
r10, r11 are caller saved.
rbx, r12, r13, r14 are callee saved
rbp is also callee saved, basepointer(can be optionally used as a frame pointer). Used to access parameters passed to the frame via offset.
rsp is callee saved, stack pointer. Only 64 or 32 bit callable, no 16 or 8 bit.
rip next instruction pointer
rsi source index, used for string operations, used with data segment as offset.
rdi destination index, used for string operations, used with extra segment (ES) as an offset

Registers only available in 64 bit size are r8 - r15

Function Argument Registers

rdi,rsi,rdx,rcx,r8 ,r9 , caller saved.
Further function args are stored inside its stack frame.

Flag Register

Status register consists of single bit flags for results of rax. It is called EFLAGS in 32 bit, in 64 bit it is called RFLAGS. Some of the flags are the following

Zero Flag, result of the last executed instruction evaluated to zero
Carry Flag, if a calculation in a register leads to a higher value than the register is capable to display, the CF would be set to 1
Sign Flag, an SF of 1 indicates a negative value, 0 a positive value
Trap Flag, 1 indicates debugging mode

Segment Registers

16 bit register, references to memory

Code Segment (CS) points to code section
Data Segments (DS) points to data section
Stack Segment (SS) points to stack
Extra Segments (ES,FS,GS) divide program's memory into four data sections

Prologue and Epilogue

Preparation begins on the stack, when a function is called. Arguments are pushed on the stack before the function is executed. The return address of the caller is pushed before the base pointer of the frame. The callers base pointer pointer is pushed and then the base pointer is to the stack pointer currents address. Afterwards the stack pointer moves with the pushed and pulled values of the function on the stack.

When the functions finished the saved base pointer is popped into the bp register and the return address is popped of to rip. The stack is set to the top of the stack once again.

Overwriting Variables and Padding

Overwrite an atomic variable behind a buffer

int main ( int argc, char ** argv ) {
    int var = 0 
    char buffer[12];
    
    gets(buffer);
    [...]
}

Stack layout

Bottom 
+------------------+
| Saved registers  |
+------------------+
| int var          |
+------------------+
| char buffer [11] |
| ...              |
| ...              |
| ...              |
| char buffer [0]  |
+------------------+
| char ** argv     |
+------------------+
| char argc        |
+------------------+
Top

Watch out! I.e., a 12 byte array is padded to system memory allocation size.

+-------------+----+
|12 byte array| 4b |
+-------------+----+
0            12   16 byte

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