Transcript sept15
CS 111 – Sept. 15 • Chapter 2 – Manipulating data by performing instructions • “What is going on in the CPU?” • Commitment: – Please read through section 2.3 – Meet here tomorrow. Basic anatomy • Inside a computer are 2 parts – CPU – Memory – These are connected by a data bus: an “HOV lane” where traffic can go either way. • CPU contains: – ALU: arithmetic and logic unit – Control unit: figures out what to do next – Registers to hold values needed for calculation • Memory (RAM) contains: – Software: list of instructions the CPU needs to perform – Data: Input and output values need to be stored while program runs Stored program idea • Program = software = list of instructions for CPU to do • Programs reside in memory • CPU will do 1 instruction at a time • For each instruction, we do the following: – – – – Fetch it from memory Decode – figure out what it means Execute – do it And then we continue with the next instruction… until the program is finished. Simple example • A program to add two numbers. • This program may reside at bytes 100-116 in RAM. • The two numbers we wish to add are located at bytes 200 and 204 in RAM. • We want the result to go into memory at byte 208. • Program may go something like this: – – – – Load the value at Memory[200] into register 1. Load the value at Memory[204] into register 2. Add registers 1 and 2, and put result in register 3. Store the value from register 3 into Memory[208]. • Note that the bus is communicating instructions (RAM to CPU) as well as data (both ways). Machine language • Unfortunately, instructions for CPU can’t be in English, French, etc. • Machine language = binary (or hex) representation of our instructions. – Each type of computer has its own machine language. • This is the oldest form of “computer programming”. Later we’ll look at much more intuitive ways to convey instructions. • Verbs: Instruction set. e.g. Add, subtract, load, store… • Nouns: Operands such as: registers, memory locations, constants, other instructions Verbs 3 kinds of instructions (instruction set) • Data transfer, using the bus – Load a value from memory into a CPU register Very similar to fetching an instruction! – Store a value from a CPU register into memory • ALU – Bit manipulation: AND, OR, XOR, NOT, shift left, shift right, … – Arithmetic: add, sub, mul, div, remainder, =, <, >, , , ≠, … • Control – “Go to” another instruction in program. In other words, interrupt normal sequence of instructions. – Can be conditional or unconditional Example language • Our book illustrates with an example HW. • 256 bytes of RAM: addressable by 8 bits • CPU contains – Instruction register – Program counter – 16 general purpose registers: addressable by 4 bits • Each register is 1 byte • Each instruction is 2 bytes = 16 bits = 4 hex digits long • Instruction format: – First 4 bits are the opcode = specify which instruction type – Other 12 bits are operand(s) • What do instructions mean? See pp. 602-603. Example instructions • Note: 16 possible opcodes: 4 bit opcode • Note: 16 possible registers: register number also 4 bits • Opcode 5 is used for adding – Expects 3 register operands – 5RST means R = S + T, where R, S and T are register numbers – Ex. 5123 means Add registers 2 and 3 and put result in register 1. • Opcode 2 is for putting a constant in a register – Expects a register operand, and an 8-bit constant operand – 2RXX means R = XX, where XX is some 8-bit pattern – Ex. 27c9 means Put the hexidecimal “c9” into register 7. • Try an example using both types of instructions. More instructions • Opcode 1 is for loading a memory value into a register – Expects a register operand (4 bits), and a memory address from which to load (8 bits). – Ex. 1820 means to go out to memory at address [20], grab the contents and load it into register 8. (It does not mean put the number 20 in register 8.) • Opcode 3 is a store = opposite of load – Ex. 3921 means to take the value in register 9, and put it into memory at location [21]. (It does not mean put the number 9 into memory location 21.) • Opcode C (hex code for 12) is for telling CPU it’s done. – Expects operand to be 12 zero-bits.