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York University
Information Technology
ITEC 1000

Introduction to Information Technology ITEC 1000 – Winter 2010 – Peter Khaiter Lecture 6 – The CPU and Memory – Feb 8 Components of the CPU - ALU (arithmetic logic unit) o Performs arithmetic and logic operations (data changed) o Arithmetic: add, subtract, multiply, divide, etc. o Logic: AND, OR, NOT, Shift, etc. o Data held temporary - CU (control unit): functions o Performs fetch/execute cycle o Accesses and retrieves program instructions from the memory and issues commands to the ALU o Moves data to and from CPU registers and other hardware components (no change in data) - Registers o Example: Program counter (PC) or instruction pointer determines next instruction for execution - Program counter (instruction pointer) o Contains the address of the current or next instruction o Normally instructions are executed sequentially - Memory management unit o Supervises fetching of instructions and data from memory - I/O Interface o Provides mechanism for input/output of data o sometimes combined with memory management unit in a single Bus Interface Unit Concept of Registers - Single storage locations within the CPU used for a particular purpose - Used to hold a binary value temporarily - Manipulated directly by the Control Unit - Each register is wired within the CPU directly (no address needed) for specific function - Size in bits or bytes (not MB like memory) - Can hold data, an address or an instruction Registers: What They Do - Hold data being processed - Hold instruction being executed - Memory or I/O address being accessed - Keep status of the computer - Conditional branch instructions General-Purpose Registers - User-visible registers - Part of ALU - Accumulators - Typically several dozen in modern CPUs (R0, R1…) - Hold data of arithmetic operations - Hold intermediate results or data values, e.g., loop counters - To transfer data between different memory locations and between I/0 and memory Special-Purpose Registers - Part of CU - Program Counter Register (PC) (instruction pointer) o Holds address of the currently executed instruction - Instruction Register (IR) o Holds the actual instruction being executed - Memory Address Register (MAR) o Holds the address of a memory location - Memory Data Register (MDR) o Holds the actual data value from location specified in MAR - Flags (one-bit register) to track special conditions like arithmetic carry and overflow, power failure, internal computer error - Status Registers o Several flag registers grouped together Register Operations - Load values from other locations (registers and memory) o Destroys (erases) previous value in destination o Source register (or memory location) unchangeable - Addition and subtraction o Result stored in the register - Shift or rotate (left or right) data - Test contents for conditions o Zero o Positive/negative o To large Program Counter - A dedicated register in the CPU - Contains the address in memory of the current instruction being executed. - Incremented automatically after each instruction. - May be forced to change: e.g. “jump” instruction. - Usually initialized to zero when machine starts, or is reset. Accumulator - A dedicated register (or set of registers) in the CPU used for the actual manipulation of data - Default source (or destination) register - Usually contains results of arithmetic or logical operations Operation of Memory - Each memory location has a unique address - Address from an instruction is copied to the MAR which finds the location in memory - CPU determines if it is a store or retrieval - Transfer takes place between the MDR and memory - MDR is a two way register MAR and MDR - Memory Address Register (MAR) o Contains Address in memory to find or place data - Memory Data Register (MDR) o Contains Actual Data to be placed in location given in MAR, or which has been retrieved from location given in MAR RAM: Random Access Memory - Random – any piece of data can be accessed in a constant time regardless of physical location (unlike tapes, magnetic or optical discs) - Difference – in technical design - DRAM (Dynamic RAM) o Most common, cheap o Volatile: must be refreshed (recharged with power) 1000’s of times each second - SRAM (static RAM) o Faster to access than DRAM and more expensive than DRAM o Volatile o Small amount used in cache memory for high-speed access ROM: Read Only Memory - Implemented in early stored-program computers (e.g., ENIAC, after 1948) - If write protected, becomes read-only memory - Non-volatile memory - to hold built-in software that is not expected to change over the life of the computer (e.g., initial program that runs computer)  BIOS: initial boot instructions and diagnostics - Data are physically encoded into chips - EEPROM o Electrically Erasable Programmable ROM o Can be erased and reprogrammed, 1 byte at a time (up to 1000 times) o Slower and less flexible than Flash ROM - Flash ROM o Modern type of EEPROM (invented in 1984), faster (erase and write in blocks of bytes) o Higher endurance (up to 1,000,000 cycles) o E.g., USB Flash Drives Memory Maps - The usage of memory space on a system is commonly depicted in a “memory map” - The height of the map is determined by the number of addresses - The width of the map is usually 8 bits o E.g. a system with a capacity of 216 bytes… Fetch-Execute Cycle - Two-step process because both instruc
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