CPU (Central Processing Unit)

The CPU contains:

• General purpose registers (about 10 or so)
  • General purpose means that the register can be used for multiple (different) purposes
  • General purpose registers are used as inputs and outputs in arithmetic and logical operations performed by the CPU
    • When the CPU add 2 values . . .
      • The first value of the addition is obtained from a general purpose register
      • The second value of the addition is obtained from another general purpose register
      • The sum is stored in a general purpose register

• An Arithmetic and Logic Unit (ALU)
  • The ALU is a complex digital circuit can perform arithmetic (+, −, ×, /) and logical (AND, OR, NOT) operations 
• A number of special purpose register
  • These registers also store binary values (32 or 64 bits)
  • They are “special purpose” meaning that the value stored in the register is used for one (special) purpose
    (I.e.: each special purpose register has a dedicated function)
  • A CPU will always contain the following 3 special purpose registers:
    • Program Counter (PC) – contains the adddress (in the RAM) of the next computer instruction that the processor will fetch and execute (after the processor finish executing the instruction in the Instruction Register)

• Instruction Register (IR) – contains the instruction (binary) code (obtained from memory) of the current instruction that the CPU is executing

• Processor Status Register (PSR) – the status code of the result after executing the last (executed) instruction
  • When the processor executes an instruction, the status of the execution may be recorded in the PSR
  • Example: if the last instruction was “x − y”, then PSR will contain status information that indicate whether x < y or x ≤ y or x > y etc…

These 3 special purpose registers together enable the CPU to

• Fetch the next computer instruction (in memory given by the Program Counter )
• Execute the (fetched) instruction

CPU will use high/low voltage signals conveyed through the bus wires to specify what operation that the CPU wants to perform

1. Low voltage ( binary number 0) – CPU issues a WRITE command to the memory
2. High voltage (binary number 1) – CPU issues a READ  command to the memory

Suppose the CPU wants to store 1010101010101010 into the memory location 4, then the CPU will emits the following electrical signals (in order):

1.  0000000000000100  (= 4 in binary) on the address bus
2.      0                 (= code for the write command) on the control bus 
3.      1010101010101010  (= the data) on the data bus

Data transfer between CPU and memory

• The CPU can read (= obtain or recall) data that is stored in the memory
• The CPU can write (= store) data into some cells in the memory
• The memory address will specify which memory cells the requested operation will be performed

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Reference

1. The structure of the Central Processing Unit (CPU). (n.d.). Retrieved July 22, 2021, from http://www.mathcs.emory.edu/~cheung/Courses/255/Syl-ARM/4-intro/cpu.html
2. Usage of the general purpose registers. (n.d.). Retrieved July 22, 2021, from http://www.mathcs.emory.edu/~cheung/Courses/255/Syl-ARM/4-intro/gen-reg.html
3. Details of the Instruction Execution Cycle explained with diagrams. (n.d.). Retrieved July 22, 2021, from http://www.mathcs.emory.edu/~cheung/Courses/255/Syl-ARM/4-intro/instr-exec-cycle2.html

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