csapp

csapp-lec-05.md

Preface

想不起太多骑行在路上时的心里想法

反正就是想去看底层浪漫网课了

想看就去做作业了

四月的时候也重新开始过，碰到周末实在是难继续

迷茫啊

每天对着脚本都快 Burn Out 了

可能最近已经经历过一轮了吧

工作效率直线下降

与其浪费时间在纠结方向上，不如都去试试

反正就是先做起来了，这回能撑多久不知道

这回没有学分，没有GPA，没有DDL，没有自己在做与众不同的事情的蜜汁快感，对未来就业没有任何直接帮助

这甚至是大一的基础课……

我甚至当年还看过……

科班出身毕业两年的人要回头去看这些东西说出来甚至是耻辱好吧

但我现在完全不会

不会就是不会

那就读个档吧

Lecture 05 Machine level Programming I Basics

assembly code == machine code

what the machine is trying to do

the core of the course

CISC Complex Instruction Set Computers

RISC Reduced Instruction Set Computers

RISC vs. CISC in 80s

C, assembly, machine code

Architecture (ISA): compiler target, parts of processor design to write assembly code

Microarchitecture: implementation of the architecture

Program counter: 

Address of next instruction

Registers

program data, heavily used

Condition codes:

store status information

Memory:

byte addressable array

code, user data

C program — Compiler —> ASM program — Assembler —> Object program — Linker —> Exceutable program

gcc -Og -S sum.c

-S, stop at
-Og, optimization at level g for debugging in recent versions of gcc
-O1, turn on the optimizer in the past, be more efficient

disassembler

objdump -d sum

referse engineers from object code to assembly code

var/func names are all lost at this assembly level, all of them are just machine code

Assembly

only do one thing per instruction

transfer data between memory and register

arithmetic functions

control flow

gdb can also disassemble function

Assembly basics: registers, operands, move

registers

%rxx, 64 bit

%exx, 32 bit, lower 32 bits of the 64 bit stuff

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%rax, %rbx, %rcx, %rdx

%rsi, %rdi, %rsp, %rbp

%r8, %r9 ... %r15

rx-abcd

r8…r15

rsi, rdi

rsp, rbp

legacy purpose: 

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accumulate

base

counter

data

si: source index

di: destination index

sp: stack pointer

bp: base pointer

move

moving data

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movq source dest

CANNOT MOVE value from MEMORY TO MEMORY

read value from memory

copy it to register

write it to memory

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D(Rb, Ri, S)

MEM[MEM(Rb) + S * MRM(Ri) + D]

naturlly work with arrays

q mean 8 bytes

bwlq, 1248

Arithmetic & logical operations

Address Computation Instruction

leaq Src, Dst

load effective address

compute address without a memory reference

compute x + k * y, k = 1, 2, 4, 8

addq, subq, imulq

salq: dest = dest << src

sarq: dest = dest >> src, arithmetic, add sign in the left

shrq: des = dest >> src, logical, add 0 in the left

xorq

andq

orq

src comes in the first

destination is the second