How does a computer work?

Shin Xiaoning Original works reproduced please specify the source
"Linux kernel Analysis" MOOC course http://mooc.study.163.com/course/USTC-1000029000

Experiment:

C Code:

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Compile command:

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Assembly Code:

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Basic knowledge

Register

registers are part of the central processing Unit. Registers are high-speed storage parts with limited storage capacity that can be used to hold instructions, data and addresses.

EIP: Stores the current instruction register, and the EIP will point to the next instruction after each execution of an instruction. Span style= "line-height:0px;" >

EBP: is the "base pointer" (base POINTER), which is most often used as a "frame pointer" (frame POINTER) for high-level language function calls. At the time of the crack, you can often see a standard function start code: < /span>

ESP: used specifically as a stack pointer, is visually referred to as the top pointer, the top of the stack is a small area of the address, the more data pressed into the stack, the ESP will become smaller. On 32-bit platforms, esp decreases by 4 bytes at a time.

EAX: Is the "accumulator" (accumulator), which is the default register for many of the addition multiplication instructions.

Addressing mode

Register addressing MOVL%EAX,%EBX assigns the value of EAX to EBX

Immediately address Movl $0x1234,%eax put 0x1234 This immediate number, assigned to EAX.

Direct addressing MOVL 0X12345,%EAX assigns the data stored 0x12345 this memory address to EAX.

Indirect addressing MOVL (%EAX), a memory address stored on%EBX eax, the value of this memory address, is assigned to EBX.

Addressing MOVL 8 (%EAX), the value stored on the%ebx eax, plus the offset 8, the location of the memory address to which this memory address is stored, the value assigned to EBX

Start with the main function:

First sentence: PUSHL%EBP

PUSHL%EBP (indicates that the contents of the EAX register overwhelm the top of the stack):

Equivalent to:

Subl $4,%esp (stacks top of stack)

Movl%ebp, (%ESP)

The second sentence: Movl%esp,%EBP

Makes EBP point to the location that ESP points to.

The third sentence: Subl $4,%esp

The top of the stack is offset by 4 bytes.

Clause Four: MOVL, (%ESP)

Put an immediate 99 on top of the stack.

Call F is called function f ()

The call command can be decomposed into

PUSHL EIP

MOVL F EIP

f function

PUSHL%EBP (Store EBP location, press stack, esp plus one)

MOVL%esp%EBP (esp points to EBP)

...

MOVL%eax (%ESP)

Point the ESP storage memory address to the EAX register

Call G Executive function g

G function

The RET function returns a function that returns the instruction within the execution Gax.

Summary: assembly language is the basis of computer languages, want to deeply understand the Linux kernel or learn some other languages, the compilation is the foundation, small Partners refueling!!!

How does a computer work?