Computer Architecture From 15 – Computer Architecture Source: OCR Computing Text Book; Hodder Education
John Von Neumann • John von Neumann (December 28, 1903 – February 8, 1957) was a Hungarian-Americanmathematician who made major contributions to a vast range of fields,[1] including set theory, functional analysis, quantum mechanics, ergodic theory, continuous geometry, economics and game theory, computer science, numerical analysis, hydrodynamics (of explosions), and statistics, as well as many other mathematical fields. He is generally regarded as one of the greatest mathematicians in modern history
Trivia • János, nicknamed "Jancsi" (Johnny), was a child prodigy who showed an aptitude for languages, memorization, and mathematics. By the age of six, he could exchange jokes in Classical Greek, memorize telephone directories, and displayed prodigious mental calculation abilities
What was said about him! • The mathematician Jean Dieudonné called von Neumann "the last of the great mathematicians",[3] while Peter Lax described him as possessing the most "fearsome technical prowess" and "scintillating intellect" of the century.[4] Even in Budapest, in the time that produced geniuses like von Kármán (b. 1881), Szilárd (b. 1898), Wigner (b. 1902), and Teller (b. 1908), his brilliance stood out.[5] From Wikipedia
Famous for.. • Von Neumann was a pioneer of the application of operator theory to quantum mechanics, in the development of functional analysis, a principal member of the Manhattan Project and the Institute for Advanced Study in Princeton (as one of the few originally appointed), and a key figure in the development of game theory[1][6]
Interested in Computer development chronology? • Check out this link • http://ysfine.com/wigner/neum/compu.html
So what is Von Neumann architecture? • It refers to • The method of processing and design of the processor that is used in most computers • Von Neumann described a processor that took each instruction serially and performed it before carrying out the next. • In addition the instructions and data were all stored together in the same memory!
So why were these concepts so revolutionary? • The concepts of • -processor processing each instruction serially before carrying out the next and • Instructions and data being stored together in the same memory.
Because • Before – in the early days of computers. • This didn’t happen! • Computers were massive, complicated things. • His ideas were at the forefront of the type of personal machines we all use today.
First there was the ENIAC
Comparison
Von Neumann wrote his famous report (see below) Concept of the “stored-program”
Computers before…. • Imagine how different they were • Von Neumann to visualise a new type of machine – one that was far more efficient. • Imagine instructions and data not stored together.
Registers – purpose and use • In order to process the instructions in a program…what does the processor require? • -A PLACE OR LOCATION TO CARRY OUT PROCESSING!
So the processor needs • a number of special locations in which specific items need to be stored while processing occurs • What are these called? REGISTERS
What are Registers? • The carry out the processing by following a specific series of actions called the FETCH-EXECUTE CYCLE
Recap • How does the processor process instructions? • It needs special locations – Registers • Registers carry out processing by following a series of actions called the FETCH EXECUTE CYCLE
Processor – is often said to be the “Brain” of the machine • Think of our brains • They fetch instructions • Decode them • Execute
Scenario: hand in fire • Touch fire • Brain receives stimulus or instruction (danger) • Instruction decoded • Man pulls away (execution)
Fetch Execute Cycle
The processor works in a similar way • FETCHES INSTRUCTIONS • DECODES • EXECUTES • But specifically –what happens?
Remember the architecture of the processor? • It includes in it things like the • PC = Program Counter • MAR = Memory Address Register • MDR = Memory Data Register • CIR = Current Instruction Register • PC = Program Counter
Let’s look at what happens in each stage of the Fetch Execute cycle in the processor
FETCH STAGE • 1. PC holds the address of the next instruction to be carried out • 2. This address is copied into the MAR • 3. The contents of the address in the MAR are copied to the MDR • 4. The contents of the MDR are copied into the CIR • 5. The contents of the PC are incremented (to point to address of next instruction)
FETCH STAGE • PC (holds add of next instruction to be carried out) • MAR (address copied into MAR) • MDR (contents of address in MAR copied in to MDR) • CIR (contents of MDR are copied into CIR) • PC (contents of the PC are incremented) Now the instruction has been “fetched” and we are ready for processing (the rest of the Fetch execute cycle!)
What comes after FETCH? DECODE
DECODE STAGE • Contents of CIR divided into binary code standing for the operation to be carried out + address of the data that will be used by the operation • CU interprets the operation code so that the processor knows what to do.
DECODE STAGE • CIR • CU Contents of CIR divided into the binary code standing for operation That needs to be carried out + the address of the data that will be used By the operation. Control unit – interprets the operation code so that the processor Knows what to do.
EXECUTE STAGE • Address part of the instruction is copied from CIR to MAR • Data found in the address in MAR is copied to MDR • The data is used!
EXECUTE STAGE • Addres part of instruction from CIR to MAR • Data found in addres in MAR copied to MDR • Data used!
Note: How data is used is quite complex. The action depends on the type of instruction. If arithmetic needs to be done then the contents of the MDR are sent to the ACCUMULATOR where arithmetic is carried out. If an unconditional jump instruction is to be carried out then the contents of the MDR are copied to the PC
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