cpu/CS and IP

I. What is CS and IP

CS and IP are two key registers in the 8086 CPU, they indicate the address of the instruction the CPU is currently reading.

CS: code segment register;

IP : Instruction Pointer Register.

In the 8086 machine, at any time, the CPU executes the content pointed to by CS:IP as an instruction.

Second, the flow chart of CPU reading and executing instructions

The following is the process of reading and executing an instruction by the 8086 CPU: (in the figure below: there is no data line from the control input and output circuit to the controller, I drew it wrong)

Figure 1: Initial state (CS: 2000, IP: 0000, CPU will go to 2000H x 16 + 0000H address read instruction)

Figure 2: The contents of CS and IP are sent to the address adder (the address adder is completed: physical address = segment address x 16 + offset address)

Figure 3: The address adder sends the physical address to the I/O control circuit

Figure 4: The control input and output circuit sends the physical address 20000H to the address bus

Figure 5: The machine instruction B8 23 01 stored from memory unit 20000H is sent to the CPU through the data bus

Note: Before the CPU reads the content to the memory (the content here is: instruction), it will issue a memory read command through the control bus, "tell" the memory, I want to start reading the content, please (memory) prepareit is good.

Figure 6 : The I/O control circuit sends the instruction B8 23 01 to the instruction buffer

Figure 7: The IP address is automatically increased (the length of the current read instruction is 3 bytes, so add 3, at this time CS: IP points to 2000: 0003)

Figure 8: The controller executes the instruction B8 23 01 (ie: mov ax, 0123H)

Figure 9: After the instruction B8 23 01 is executed, the content in AX is 0123H

After: The CPU will read the instruction from the memory unit 2000:0003 and cycle the above steps in turn.

The following is a dynamic diagram of CPU read instructions.

III. Summary

1. The CPU reads the instruction from the address pointed to by CS : IP, and the read instruction enters the instruction buffer;

2, IP = IP + the length of the instruction read, so as to point to the next instruction;

3. Execute the instruction.Go to step 1 and repeat the process.

Reprint: CS and IP - Knowing

(SAW: Game Over!)