One of the basics of assembly language-Introduction to CPU architecture and register types

Intel X86 architecture

The Pentium i-386 is a 32-bit CISC (Complex Instruction computer) processor, but the aplha is based on Risc (short instruction computer ).CodeBut the principles are similar.

The processor has many General registers. Registers are storage locations that are of special use in the microprocessor itself. Access registers are much faster than accessing other types of memory. Some registers are common, while others are of special use.

General registers are used to store the values used for calculation and internal calculation results. In general, the registers manipulate byte, word, or DWORD.

The flag register is only used to manipulate the bit. Any instruction affects the flag register based on the calculation result. For example, after a subtraction operation, if the result is 0, zero flag will be set. If it is not 0, its value will be cleared.

An assembly instruction can have 0, 1, and 2 operands. The operand can be an immediate number, a value in the register, or a pointer to an address in the memory. Generally, the Intel assembly language syntax is as follows:

Prefix instruction [operand 1], [operand 2]; Comment

Where

Prefix	-An address label or instruction Modifier	Prefix. ID address or operation Modifier
Instruction	-Instruction being executed	Command name
Operand 1	-Typically the destination operand, but it can be the source operand (one argument instruction), or it can be both the source and destination operands (INC instruction)	Operand 1. In general, this operand is the destination operand.
Operand 2	-Source operand for two argument instructions	Operand 2

 

Therefore, for the mov eax, [12345678] command, the source operand is in address 12345678, And the destination operand is the eax register. This command will drop the value stored in 12345678 and copy it to the eax register.

 

Intel CPU registers

The intel386 and later processors contain a series of 32-bit registers. The i386 Family of processors are considered non-orthogonal, meaning that registers and instruction sets cannot be completely exchanged. That is to say, some commands can only be used in specific registers. For example, the in and out commands are hardware-fixed and can only be used in the eax registers. ECx is used for cyclic counters, and EDI and ESI are used for index commands and string commands. The addressing mode can only be used in specific registers.

The i-386 family has six General registers, eax, EBX, ECx, EDX, EDI, ESI. each register starts with the letter E, indicating extended, which means to extend to 32 bits, not sixteen bits.

Usage

Al	Lower 8 bits of the eax register	Eax's eight-bit low
Ah	High Order byte of lower order word of the eax register	Eax's 8-bit high
Ax	Lower 16 bits of the eax register	The lowest 16 bits of eax
Eax	The full 32 bits of the Register	Entire eax register

EBX and ECx are the same as edX.

Note that ESI and EDI cannot be used at the byte level.

 

Register Type

General registers

1. eax is used as a accumulator. It is the most used in registers and is used to save the results of many commands. Generally, compiled code only uses the eax register to store the return value.

2. ECx is used as a counter.

3. EBX and EDX are General registers and are generally used as pointers for memory addressing, arithmetic operations, logical operation operations, and operation results of commands. Eax and ECx can also be used like EBX and EDX.

Index register

EDI and ESI are General registers dedicated for indexing. String operations use EDI as the destination pointer and ESI as the source pointer. Therefore, if you want to copy a piece of memory from one place to another, ESI should be used as the source block and EDI as the target block. ECx should load the number of bytes to be copied. The direction flag will be set to increase or decrease, and then the rep movs command will copy the bytes.

Stack register

ESP and EBP are mainly used to manipulate the control station. ESP is the top pointer of the stack and is used to point to the top position of the current stack. The EBP register is used to point to the stack frame, that is, the bottom of the stack, for a given routine (function. At the routine (function) entry, the EBP register is usually first stored on the stack (the EBP is pressed to the stack), and then set EBP to the top pointer (ESP) of the current stack ). EBP is used to reference parameters or local variables. Local variables can be referenced by EBP, [EBP-4], [EBP-8] and so on. Parameters can also be obtained through EBP, for example, [EBP + 8]. Generally, local variables are in the Negative displacement of EBP, while function parameters are in the positive displacement of EBP.

Note that ESP points to the address with a value in the stack, that is, esp points to the valid byte at the top of the stack. Let's explain it in combination with the stack operation. The operations in the following table are equivalent.

Push 0x1234 h;

ESP <-ESP-4; reduces the position of the stack pointer SS: [esp] <-0x1234 h; copy the operand to the top of the stack.

 

The following code is of reference value.

Void someprocedure (INT anargument) {int avariable; avariable = anargument;} someprocedure: Push EBP; save original value of EBP on Stack mov EBP, esp; store top of stack address in EBP sub ESP, 4; Allocate space for avariable on Stack mov eax, [EBP + 8]; fetch anargument into eax, Which is; 8 bytes below the stored top of stack mov [EBP-4], eax; store eax into avariable, Which is; 4 bytes abve the stored top of stack mov ESP, EBP; free space allocated for avariable pop EBP; restore original value of EBP ret; return to the caller
 

Flag register

It consists of a series of individual BIT. Many Commands modify these binary bits to describe the results of the command. These flags can be used by conditional jump JMP statements.

For more information, see the table below.

Intel x86 flags register
Bit #	Abbreviation	Description	Category[1]
Flags
0	Cf	Carry flag	S
1	1	Reserved
2	PF	Parity flag	S
3	0	Reserved
4	AF	Adjust flag	S
5	0	Reserved
6	ZF	Zero flag	S
7	SF	Sign flag	S
8	TF	Trap flag (single step)	X
9	If	Interrupt enable flag	X
10	DF	Direction Flag	C
11	Of	Overflow flag	S
12, 13	Iopl	I/O privilege level (286 + only)	X
14	NT	Nested task flag (286 + only)	X
15	0	Reserved
Eflags
16	RF	Resume flag (386 + only)	X
17	VM	Virtual 8086 mode flag (386 + only)	X
18	AC	Alignment check (486sx + only)	X
19	VIF	Virtual interrupt flag (Pentium +)	X
20	VIP	Virtual interrupt pending (Pentium +)	X
21	ID	Identification (Pentium +)	X
22	0	Reserved
23	0	Reserved
24	0	Reserved
25	0	Reserved
26	0	Reserved
27	0	Reserved
28	0	Reserved
29	0	Reserved
30	0	Reserved
31	0	Reserved
Rflags
32-63	0	Reserved

Table note:

S: Status flag

C: Control flag

X: System flag

Special destination register

The EIP is the instruction pointer that points to the next instruction to be executed. The EIP is changed by the command ret, reti, JMP, call, and Int. This register may be the most important register in the CPU, because it guides the CPU to execute the next instruction.

Segment register

In 32-bit systems, segment registers are generally not modified or explicitly used. Win32 uses the flat memory addressing mode. For this reason, seeArticleThe description in "memory arrangement for Windows" in "one of Windows backgrounds.

Additional registers

There are some other registers in the Intel family, but they are not suitable for user-state debugging. A simple list of them is as follows:

Protected Mode registers

Control registers

Debug and test registers

Floating Point Unit register set

MMX registers

SIMD floating point registers

 

Exercise:

1. What are the main functions of the eax register?

A: eax is used as a accumulator. It is the most used in registers and is used to save the results of many commands. Generally, compiled code only uses the eax register to store the return value.

2. What are the main functions of ECx registers?

A: Counter

 

Tips for helping with memory

Register	Full name	Description
Eax	Extended accumulator x	Accumulate registers. A Indicates Accumulator
ECX	Extended counting x	Count register. C stands for counting
EDI	Extended destination Indexing	The destination index register. D Indicates destnation, And I indicates indexing.
ESI	Extended source Indexing	Source index register. S stands for source, and I stands for indexing.
ESP	Extended Stack pointer	Stack pointer register. S stands for Stack
EBP	Extended (stack) base pointer	Stack base pointer register. B stands for base
EIP	Extended instructions pointer	Command register. I stands for instruction