[MCU] 2.3 CPU of AT89S52

2.3 AT89S52 Of CPU

• CPU from Arithmetic unit and controller constitute .

2.3.1 Arithmetic unit

• Arithmetic on operands 、 Logic and bit operations . It mainly includes arithmetic and logic operation unit ALU、 accumulator A、 Bit processor 、 Program status word register PSW And two registers .

1． Arithmetical logic unit ALU

• But for 8 Bit variable logic operation （ And 、 or 、 Exclusive or 、 loop 、 Make up and clear ）, Still can Arithmetic operations （ Add 、 reduce 、 ride 、 except ）
• ALU also Bit operation function , Bit processing of bit variables , If you set “1”、 clear “0”、 Make up 、 Test transfer and logic “ And ”、“ or ” etc. .

2． accumulator A

• The most frequently used register , Can be written as Acc.“A” And “Acc” Differences in writing , In the fourth 3 Chapter introduction . Special function register area located in the chip .
• It works as follows ：
  * （1）ALU One of the input data sources of the unit , again ALU Storage of operation results unit .
  * （2） Data transfer Mostly through accumulators A, Equivalent to a data transfer station . To solve the problem “ The bottleneck is blocked ” problem ,AT89S52 Added a part of transmission instructions that can not pass through the accumulator .
• A Carry flag of Cy It's special , Because it is also Bit accumulator of bit processor .

3． Program status word register PSW

• PSW（Program Status Word） Located in the on-chip special function register area , Byte address is D0H.
• Contains Information about the running status of the program , among 4 Bit saves the status of the current instruction after execution , For program inquiry and judgment . The format is as shown in the figure 2-5 Shown .
  
• PSW Functions of each bit in ：
  • （1）Cy（PSW.7） Carry flag bit
    • Can be written as C. In arithmetic and logical operations , if Have carry / Borrow position ,Cy＝1; otherwise ,Cy＝0. In the bit processor , It's a bit accumulator .
  • （2）Ac（PSW.6） Auxiliary carry flag bit
    • stay BCD Code operation , Used as a decimal adjustment . When D3 Orientation D4 When a carry or borrow occurs ,Ac＝1; otherwise ,Ac＝0.
  • （3）F0（PSW.5） The user sets the flag bit
    • A status flag bit used by the user , Instructions can be used to set it 1 Or Qing 0, Control the flow direction of the program . Users should make full use of .
  • （4）RS1、RS0（PSW.4、PSW.3）4 Group work register area selection
    • Select on-chip RAM In the district 4 A group in the group working register area is the current working register area, as shown in table 2-2.
      
  • （5）OV（PSW.2） overflow flag
    • When executing arithmetic instructions , Used to indicate whether the operation result overflows . If the result is overflow ,OV=1; otherwise ,OV=0.
  • （6）PSW.1 position ： Keep a
  • （7）P（PSW.0） Parity flag bit
    • Command execution completed , accumulator A in “1” Whether the number of is odd or even .
      • P=1, Express A in “1” The number of Odd number .
      • P=0, Express A in “1” The number of even numbers .
    • This flag bit is of great significance for serial communication , Commonly used Parity test To test the reliability of data serial transmission .

2.3.2 controller

• Mission ： Identification instruction , And according to the nature of the instructions, control the functional components of the single chip microcomputer , So as to ensure that all parts of the single chip microcomputer can work automatically and harmoniously .
• The controller includes ： Program counter 、 Instruction register 、 Instruction decoder 、 Timing and control logic circuit, etc . The function is to control the reading of instructions 、 Decoding and execution , Thus, the timing and logic control of each functional component .
• Program counter PC It's an independent 16 Bit counter , inaccessible . When the MCU is reset ,PC The content of 0000H, From program memory 0000H Unit fetch instruction , Start the program .
• PC working process ：CPU When reading instructions ,PC As the address of the fetched instruction , The program memory outputs instruction bytes at this address , meanwhile PC Automatic addition 1.
• PC Content change track in Determine the process . When Sequential execution Automatically add 1; perform Transfer program or Subroutines 、 Interrupt subroutine call when , Automatically change its contents to the destination address to be transferred .
• PC Count width of Determines the address range of the program memory .PC by 16 position , Therefore, we can **64KB（=2^16B）** Addressing .