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Department
Information Technology
Course
ITEC 1000
Professor
All Professors
Semester
Winter

Description
Introduction to Information Technology ITEC 1000 – Winter 2010 – Peter Khaiter Lecture 7 – Instruction Addressing and Programming Tools – Feb 22 Direct, Absolute Addressing − Direct: data is reached directly from the address in the instruction − Absolute: address in the instruction field is the actual memory location being addressed Additional Addressing Modes − Programmer-accessible registers o Provide faster execution with register-based instructions − Alternatives to absolute addressing o Allow larger range of addressable memory o While using a reasonable number of bits for the address field − Alternatives to direct addressing o Facilitate writing certain types of programs o Example: loops that use index to address different entries in a table or array Register Addressing - Does not require a memory access o Contents of the source register is directly moved to the destination register o Faster execution as memory is not accessed o Practical application: frequently used data to be loaded from memory into registers and left there - Implemented directly as part of the CPU o All registers are located in the CPU (as a part of ALU or a separate register unit) Two Alternatives to Absolute Addressing - Base register addressing - Relative addressing - Both provide starting address and an offset or displacement from the starting point o Starting address in register or program counter o Offset: address in the instruction - Programming advantage: permits to move the entire program to a different location in memory without changing any of the instructions (i.e., relocatability) Base Register Addressing - Base register set to initial address o Hardware design: special/separate register or general-purpose registers o Generally large to provide large memory space, frequently gigabytes - Final address: contents of instruction address field added to the base address Relative Addressing - Value in address field added to value in program counter o Program counter used as the base register o Similar to base addressing - Constraint: address field must be able to store and manipulate positive and negative (to jump backwards in loops) numbers o Complementary representation Direct Addressing - Location of data is different from location of instructions - Benefits to programmer o Data can be changed without affecting the instruction itself o Data is available to different instructions o Variable can be assigned to a particular location in memory independent of the instructions that refer to data Alternatives to Direct Addressing - Immediate addressing - Indirect addressing - Register Indirect addressing - Indexed addressing Immediate Addressing - Store data with the instruction itself o Example:  Data is a constant - Constraint: o Address field must be able to store and manipulate positive and negative numbers o Complementary representation o Instruction must include extra bits to identify the addressing mode o Change instruction if the constant has to be changed - Advantage: o Additional memory access (to obtain the data) is not required o Faster execution Incrementing - Treat the instruction as data o Modify the address field - Pure code: does not modify itself during execution o Incrementing does not modify the instruction o Address stored in a separate data region o Advantage: program can be stored in ROM Register Indirect Addressing - Also called register deferred addressing - Address pointer is stored in a general-purpose register - Advantage: efficient o One instruction to load pointer address in register o Data accessed in the same number of fetch-execute instructions as direct addressing o Small address field required (3 or 4 bits) o Excellent for addressing large memory space with small instruction word Register Indirect Addressing: Duel Duty - Autoincrementing/autodecrementing o Direct implementation of C’s “++” and “- -” - Instruction o Performs normal functions like LOAD or ADD o Plus increments or decrements register each time instruction executed - Advantage: simplifies writing program loops Indexed Addressing - Use address in the instruction like direct addressing o But modify address by adding value from another register o General purpose or special index register o If the value in index register is 0, index addressing becomes direct addressing Indexed vs. Base Offset - Both offset address by amount stored in another register - Base offset: primarily to expand addressing range for a given address field size o Value of base address likely to be large and rarely changed during execution - Index register: primarily used as a table offset for subscripting o Value in index register most like small and frequently changing o Autoindexing: similar to autoincrementing (except that the index register is incremented) Programming Tools - Editors – entering and modifying program in text form - Assemblers, interpreters, compilers – translating a program into binary machin
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