Playing in Assembly

Playing with Assembly to make my inner child happy!

References

Some Assembly Required
Understanding Windows x64 Assembly - For help getting this running on Windows since the guide/code was written for Mac

Thinking in Assembly

Control flow is with compare and jump. Symbols: cmp, jump, jl

Math

Convert binary "1011001" to base 10:

1 * 2^0 = 1
0 * 2^1 = 0
0 * 2^2 = 0
1 * 2^3 = 8
1 * 2^4 = 16
0 * 2^5 = 0
1 * 2^6 = 64

1011001 (base 2) = 89 (in base 10)

Base can be any number:

Base 2 - binary
Base 10 - decimal, our typical number system
Base 16 - hexadecimal or hex for short

Using Programming calculator is really nice!

Logic Gates:

AND - always false unless both inputs are true
OR - always true unless both inputs are false
NOT - flips the input
NAND - always true unless both inputs are true, opposite of AND
NOR - always false unless both inputs are false, opposite of OR
XOR - true if the inputs are different, false is they're the same
XNOR - true if inputs are the same, false, if they're different, XNOR is the opposite of XOR

Dictionary

CPU

CPU - central processing unit, can only do a few things (1) Read data (2) Write data (3) perform simple math calculations via logic gates, everything in the CPU is just numbers, CPU also have a set of instructions physically built into the chip used for opcodes, a 64-bit CPU can hold 64 bits of data in it's registers which is 8-bytes
RAM - Random Access Memory, short-term (ephemeral) data that has a much larger capacity than the CPU's registers
Register - CPU's small storage, generally you get around 16 general use registers depending on the CPU, there are more than than but they are only used internally by the CPU
Instruction Categories - (1) opcodes, add (2) numeric value, 4 (3) letter, "A" (4) register, rax (5) memory address, 0x12345678
Processor clock - a material that oscillates electricity running through the CPU at a certain frequency giving the CPU a "clock tick" which keep the CPU in sync as each tick is an execute of another instruction, today's CPU are measured in gigahertz (GHz) which are approximately one billion cycles per second

Instruction Cycle

Fetch - grabs data from RAM
Decode - translates the data into an instruction CPU can understand, the first part of the data is the opcode and the rest are arguments
Execute - performs instruction, if arithmetic or logic instruction then performs extra work in the ALU (arithmetic-logic unit)

opcode - unique identifier for an action the CPU can execute, add which is really a number 4 (not actually 4)
arithmetic-logic unit aka. ALU - performs math or logic and returns value to register

Data

bit - one unit of data, a 1 or 0
byte - a group of bits, the smallest collection of bits, 8 bits, fun fact it's 8-bits because of ASCII fitting all characters in the 255 size (technically a nybble is smaller but it isn't used often)
word - group of bytes, single word is 2 bytes, double word is 4 bytes, and quadruple word is 8 bytes
qword - shorthand for quadruple word

Registers

Usually, 32-bit processor registers start with e (eax) whereas 64-bit processors start with r (rax)

General purpose but some instructions automatically set these:

rax - accumulator
rbx - base
rcx - counter
rdx - data
rdi - destination
rsi - source

General purpose:

r8 - user defined
r9 - user defined
r10 - user defined
r11 - user defined
r12 - user defined
r13 - user defined
r14 - user defined
r15 - user defined

Auto-set:

rbp - stack base pointer
rsp - stack pointer
rip - instruction pointer
eflags - status / condition

Stack

A place to store data for later use, last-in first-out data structure.

Push - place a register value in the stack on top of all the previous values
Pop - retrieve a value from the top and put it in a register

Conventions for x86-64:

Arguments get pushed on the stack to "pass" into the "function"
Arguments get popped off the stack and into registers to access inside of our "function" (stored in r8-r15)
"Return" values get saved in rax
General purpose registers can be "callee-owned" or "caller-owned"

Syntax

mov dest, source - store data in a register
add dest, source - addition, dest + source = stored in dest
sub dest, source - subtraction, dest - source = stored in dest
mul dest, source - multiplication, dest * source = stored in dest
jmp label - moves the instruction pointer via a label .addNumbers
cmp dest, source - compares two values and stores results in eflags register
jl label - stands for, jump if less than, by looking in the eflags register
call label - how assembly implements functions combined with ret
ret - returns to line after call
push source - store data on the stack and increment the stack pointer
pop dest - retrieve data off the stack and decrement the stack pointer

buwilliams/Learn_Assembly_x86-64