Computer System Ch.3

Procedure

An abstraction mechanism that encapsulates code with a set of parameters and a return value, allowing execution control and data flow to transfer cleanly to another part of the program.

x86-64

An instruction set architecture: the instructions a processor understands, its registers, and the rules for using them. It is the interface between software and hardware.

Also written x64, AMD64, or Intel 64

• In stack, Grows toward lower addresses

Every call solves three problems.. 1? 2? 3?

1. Passing control

2. Passing Data

3. Allocating Memory

Register

A small storage location inside the processor itself, usually 64 bits(8 bytes). A processor has about sixteen, and reading one is far faster than reading main memory.

%

Punctuation only. It tells the assembler that a word is a register name

• “rax” , etc.

Run-Time Stack

A contiguous region of memory organized as a stack that manages procedure calls, parameter passing, return address, and local storage.

If P calls Q, __ must finish before __continues

Q must finish before P continues. The most recent call is always the first to complete. — LIFO rule

Stack Frame

The private region of the stack belonging to one invocation of one function.

• A portion of the run-time stack allocated specifically for a single procedure execution, bounded by the stack pointer %rsp (and option a frame pointer %rbp )

Stack Pointer (%rsp)

A dedicated x86 - 64 register that holds the memory address of the top of the stack.

Control Transfer

The mechanism by which execution branches to the beginning og a called procedure using call and returns to the calling code using ret.

Return Address

The address iof the instruction immediatley following a call instruction, which is pushed ontot he stack so the CPU knows where to resme execution upon returning.

Calling conventions registers

Registers (%rax , %rdi, %rsi, %rdx, %rcx, %r8 - %r11 ) whose conents may be overwritten by a called procedure

Callee-Saved Registers

Registers (%rbx, %rbp, %r12, %r15 ) whose contents must be preserved across procedure calls

Array

A contiguous region of memory storing a sequence of homogeneous elements, accessed using zero-based indexing.P

Pointer arithmetic

An operation where adding or subtracting an integer i to a pointer automatically scales i by size Iin bytes) or the underlying data type ( Address = Xa + L * i)N

Nested Array (Multidimensional Array)

An array whose elements are themselves arrays major order as single contiguous memory blow

Row-Major Order

A memory layout strategy where multi-dimensional array elements are stored row by row in contiguous memory locations.

Fixed-size array

An array whose dimensions are known and fixed at compile time, allowing the compiler to optimize indexing computations into direct address calculations.

Variable-size array (VLA)

An array whose dimensions are determined dynamically at run time, requiring extra arithmetic (or scaling registers) to calculate element address.

Structure (struct)

A C data structure that groups elements of potentially different data types into a single contiguous block of memory, storing fields at fixed byte offsets relative to the start of the structure.U

Union (union)

A C data structure that allows a single block of memory to be referenced using multiple different data types; its total size equals the size of its largest member, and all share the same starting address offset (0)

Data Alignment

A hardware placement requirement where object memory address must be multiple of a specific value K (typically 2, 4, or 8) , ensuring efficient memory bus transfers and preventing performance penalties or bus errors.P

Padding

Unused bytes inserted by the compiler between structure fields or at the end of a structure to satisfy data alignment constraints.

Call

Pushes the return address - the address of the instruction immediatley after the call - onto the stack, so %rsp decreases by 8, the size of one address. Then sets %rip to the address of the called function.

ret

Pops the return address of the stack into %rip, so rsp increases by 8, and execution resumes in the caller at exactly the instruction after the call.

Passing data : arguments and return values

They are assigned in a fixed order: first argument in %rdi, second in %rsi, and so on. Arguments seven and beyond go on the stack, padded to a multiple of 8 bytes, just above the return address: arg 6 16(%rsp) argument 8 8(%rsp) argument 7 0(%rsp) return address The return value is always placed in %rax.

A functions needs space only in three situations 1?2?3?

1. There are more local values than available registers.

2. The address operator & is applied to a loccal variable, so it must occupy a real memory address.

3. The variable is an arrray or a structure, which cannot fit in a single register.

How does x86-64 prevent recursive function calls from overwriting earlier calls' local variables?

Each function invocation receives its own separate stack frame allocated at a lower memory address, ensuring every call maintains distinct memory for its parameters and local variables.

In what order are stack frames allocated and deallocated during a recursive call sequence?

Stack frames accumulate (grow downward) as recursion descends toward the base case, and are deallocated in reverse order (LIFO — Last In, First Out) as each call returns.

Complete Sequence Order (Function or procedure call in C/Assembly)

1. Caller prepares

2. Call executes

3. Callee executes

4. ret executes

Push

The call instruction pushes the return address onto the stack.

Jump

The CPU jumps to the called function’s code C

Compute

The function executes its work/math on theCPU.

Pop

The ret instructions pops the return address of the stack

Jump

The CPU jumps back to the caller’s code to resume execution.

Heterogeneous

Made of different types. An array is homogeneoues - every element has the same type.

Field

One named value stored inside a structure or union, such as grade or score.

Offset

The distance in bytes from the start of the structure to the beginning of a field.A

Alignment

The rule that a value must begin at an address that is a multiple of its own size.

Pointer Size on 64-bit Architecture

Always 8 bytes

• char* , int* , struct *

&x

Address-of-operator(produces memory address of x)

*p

Dereference operator(access “the value stored at that address”)

Pointer Arithmetic Scaling

Adding 1 (p+1) advances the pointer address by sizeof(*p) bytes. NOT 1 byte

Pass by value in C

Passes all function parameters by value

void *

A pointer type with no associated element size.

• Cannot be dereferenced or used directly in arithmetic without explicit casting.

• 
  

argc

Argument count

argv[0]

Holds the name of the exacutable program

sizeof

A compile-time operator that returns the total size in bytes of a type or variable, including any padding bytes added for alignment

typedef

Creates a new alias/name for an existing data type. It creates no new data type and does not change memory layout.

break main / break file.c:20

Set a breakpoint at a function or a line.

run

Start the program

continue

resume after a stop

next

next runs the next line over any call

step

step goes into the call.

print x

Shows a value

print &x

show its address.

print sizeof(struct Employee)

Confirm a struct size, padding included.

print &e.score

Confirm a field offset — subtract &e from it.

x/12xb &e

Examine 12 bytes in hex starting at the address of e — this is how you see padding.

info locals

Show local variables

info registers

show register contents including %rsp and %rip.

backtrace (bt)

Show the chain of stack frames

finish

Run until the current function returns, and show its return value.