Processor Fundamentals

Von Neumann Model

• Modern Computer system architecture
• Stored Program: Computer Program and data are stored in main memory rather than hard-wired in hardware
• The CPU consists of:
  • Registers
  • Control Unit (CU)
  • Arithmetic Logic Unit (ALU)

Run the "Addition" example in LMC, and write down the functions of:

• Arithmetic Unit
• Program Counter
• Instruction Register
• Address Register
• Accumulator

Registers

• Storage unit within the processor
• Fastest memory in a computer system
• Very small in size (each maybe a few bytes)
• General purpose registers
  • ​store data values from main memory or results of processing
  • can be used by programmers
• Special purpose registers​

* https://peterhigginson.co.uk/LMC/

Buses

• Connects components within computer system
• Including:
  • Address Bus
    • Carries address to memory, for identifying a location in the main memory
  • Data Bus
    • Carries data from main memory to processor, input and output devices
  • Control Bus
    • Carry type of control signal will occupy one wire(line) in the control bus
    • e.g. there will be a wire for "interrupt"
    • e.g. another wire will represent memory write completed etc. 

Diagram showing some buses with wires in a computer system

w17/qp13/4

Interrupt

• A signal to the processor generated by hardware or software
• to flag that some event has occurred 
• Carried in the Control bus - Each interrupt will occupy one wire in the bus
• e.g. 
  • When there's Keyboard input
  • Printer sends message when it's out of paper
• CPU will run some code to handle the event, usually pausing the current task
• Resume the task once interrupt is handled

Performance of a PC

• The "speed" of a computer is determined by many factors
• Some of those are:
  • Clock speed
  • Bus Width

Clock speed

• The CPU will time it's operation with a "clock", which "ticks" in a set period of time
• e.g. modern desktop CPU is running at about 2GHz -> 2 Billion ticks per second
• Each operation will require a certain clock cycle to complete it
• Note the clock cycle required is different for different types of CPU, but for the same CPU architecture, the clock cycle required are fixed
• So one way to determine speed of CPU is the clock speed

Bus Width

• A Bus contains a number of physical wires
• Parallel data transmission: Each wire carries one bit at the same time
• e.g. If a CPU has 32-bit data bus and 16-bit address bus
  • The largest data each time it can transfer is 2^32
  • That also means the largest memory block it can address is 2^16
  • Assume 1 block is 1 byte, how large is the main memory?

Fetch-execute cycle

Refer p.87 for the flowchart, also LMC to understand more on this

Processor instructions

• Machine code is the instruction that directly decoded by CPU, and denotes as binary values
• Each machine code can break into Opcode (what operation) and Operand
• Assembly Language translates (almost) 1-to-1 machine code by Assembler

Assembly

LDM #33

Machine Code

0000 0001 0010 0001

Opcode

0000 0001

Operand

0010 0001

Modes of addressing

Absolute vs Symbolic addressing

• All addressing values in the machine code is the actual address number
  • e.g. LDD 30 will load the data in address "30"
• We can use "symbol" to represent a physical address of the location, e.g. LDD value
  Then the assembler will calculate the offset  of this symbol from the start of the program and replace with actual address

Directives

• Code inside assembly file, but not actual program instruction
• Directive is instruction to the assembler and linker
• e.g.
  • ORG 100 - Tell the assembler the program code starts from address 100, instead of 0
  • INCLUDE "FILE.LIB" - Include extra file in the program

Macros

• Macro is similar to "procedure" where code can be reused with different supply parameters

;Define a macro called "NameOfMacro"
;Note the keyword MACRO itself is directive
MACRO NameOfMacro
    <instructions>

END MACRO

;Calling macro
NameOfMacro

Assembler

• Assembler "translates" assembly code into executable machine code
• Two-pass assembler:
  • first pass
    • assign address to all statements in the program
    • Address of symbolic labels (e.g. variable) are stored
  • second pass
    • translate opcode and operands

First pass

• Uses a Symbol table to store name of symbols and their respective absolute address
• Codes are parsed line after line, and the symbol table is updated accordingly
  • Label will be added / updated in table with the address of it
  • Operand using labels will be added to table with unknown address
• Link to the simulator:
  https://replit.com/@andytsuiacm/2-pass-assembler

Second Pass

• Opcode will be translated into machine code using the Operation code table
• Operand with address label (symbolic link) will be translated to absolute address using the Symbol table done in the first pass