Chapter II Register

First, CPU overview

1, a typical CPU by the operators, controllers, registers and other components.

2. The internal bus realizes the connection between each device inside the CPU .

3. The external bus realizes the connection between the CPU and other devices on the motherboard.

Second, the General register

1, 8086CPU are 16 bits can be stored two bytes. Ax. Bx. CX. DX is typically used to store general data, known as general-purpose registers.

2, in order to ensure compatibility, these four general-purpose registers can be split use. For example, Ax: can be divided into Ah, AL.

Three, the word in the Register of storage

1, Bytes: Byte, a byte consisting of 8 bits, can hold 8-bit registers.

2, Word: recorded as Word, a word consists of two bytes, respectively known as the high byte of the word and status byte.

Iv. several assembly instructions

1.

1) in the Mov and add instructions, the number of bits of the two operand should be the same.

2) When the operation exceeds the Register, the high position is omitted. (1 omitted)

Five. 8086 represents a physical address barrier

The 1.8086 registers are 16 bits (64KB) and the external address bus bits are 20 bits (1M).

2. Therefore 8086 in-house with two 16-bit addresses by calculating the physical address of a 20-bit can be synthesized

The computational composition is implemented by the address adder inside the CPU.

3. Concept of paragraph

"segment" is a logical concept introduced in 8086 to address the physical address of a 20-bit with a 16-bit register.

You can use a set of internal CDs (no more than 64KB) as a "segment", where the address is contiguous and the starting physical address is multiple, as needed.

VI, CS and IP

1, CS: Code segment register, for storing the "segment address" of the code snippet

2. IP: instruction Pointer register, which holds the offset address of the instruction that the CPU will read in the code snippet.

3,Cs:ip corresponding to the physical address of the CPU will be read instructions.

4. Modify CS and IP instructions in jmp

1) simultaneously modify CS, IP content:jmp segment address : offset Address

Modify CS with the segment address given in the instruction, offset address to modify IP.

2) modify only the contents of the IP: a valid register injmp

Chapter II Register