Assembly Primer

Assembly Primer

CPU Registers

• Little "hardware variables" very close to the CPU that are blazing fast

CPU Instructions

• Single purpose "functions" that operate on/with CPU registers

• They can perform arithmetic, moving values etc

Memory

• A continuous, volatile space of "storage slots" (words) that can be addressed by the CPU

• It's mostly where your program lives when it's running

Assembly Primer

CPU Registers

• Specific to each architecture

• x86_64 uses 16 general purpose registers

Assembly Primer

CPU Instructions

• mov (Move - Copies the data refered by one item to the other)

• Push (Places stuff on top of the stack)

• pop (Removes stuff from the top of the stack)

• ...

Data Movement

• add (Integer addition)
• sub (Integer subtraction)
• inc, dec (Increment/Decrement)
• ...

Arithmetic

• jmp (Jump - Transfers flow to the instruction at a memory location)
• jCondition(je/jne/... - JumpWhenEqual/JumpWhenNotEqual according to the "machine status word"
• cmp (Compare - Sets the "machine status word" according to the comparison
• ...

Control flow

Assembly Primer

Memory

Assembly Primer

Memory

Stack

• LIFO Datastructure

• Usually grows downwards to lower addresses

• Stores all the data required by a function call

  • Everytime a new function is execute, a new stack frame is being created at the top of the stack

  • When it finishes, the stack from is poped from the stack

  • The stack frame contains all the functions parameters, the return address back to the caller of the function and any variables local to that function.

Assembly Primer

Memory

Heap

• A memory segment for dynamic memory allocation

  • All reference types created by malloc/new 

  • Uses the brk and sbrk system calls to adjust its size

• Heap is used:

  • Memory size is dynamically allocated at run-time 

  • Scope is not local to a function

  • Allocated size is large

Assembly Primer

Memory

BSS (Block Started by Symbol)

• An unitialized data segment

• Data in this segment is usually initialized by the kernel before the program starts its execution

• I.e static let index = 0 will be allocated in the BSS

Assembly Primer

Memory

Data

• This segment contains initialized global and static variables which have a pre-defined value and can be modified.

• It has a read-only and a read-write space.

• Let's just say it holds global variables

Text

• A segment where basically assembly instructions are stored.

• This is read-only

Assembly Primer

Memory

How stack frames work