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Date: Tue, 05 Nov 1996 20:51:06 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Fri, 16 Aug 1996 15:35:26 GMTContent-length: 8475<html><head><title> Lecture notes - Chapter 1 - Some basics.</title></head><BODY><h1> Chapter 1 -- Some basics</h1><pre>ADMINISTRATIVE STUFF:introduce selfgoals for course: provide basic understanding for future courses an introduction to computer architecture learn low level details about how computers workhand out: policy sheet lab acct. forms first assignment academic misconduct policy form brochuretext at bookstorego over policy sheet in detailgo over academic misconduct in detailUnix Tutorialslog in soon! Some introductory material--------------------------basic concept within computer science and engineering: Levels of Abstraction hierarchy models used (for our purposes) to design programs, computers when a problem is large, it needs to be broken down -- we "divide and conquer" one way is by introducing a hierarchy (level), and solving the problem at each level example: design of a computer 1. transistors available 2. gates, flip flops 3. components, like registers and adders 4. CPU, memory system, I/O devices 5. computer system | ------- CPU | computer system ------| ------- memory | | ------- I/O stuff components <----> gates <--------> transistors TOP-DOWN vs. BOTTOM-UP design another example: software levels of abstraction writing a large program -- >10,000 lines of code TOP-DOWN: divide into modules, design each module separately. define procedures (functions) that accomplish a task. specify interfaces (parameters) for the procedures. the implementation of a function is independent of its interface specification! it is a different level in the abstraction of program design. the "big picture" -- a computer with software running on it. HLL computer Pascal, C, Fortran . . . . . . . . . hardware how do we get from one to the other? wanted: write in nice abstract HLL. have: stupid computer that only knows how to execute machine language what is machine language? binary sequences -- interpreted by computer as instructions. not very human readable. to help the situation, introduce assembly language -- a more human readable form of machine language. uses MNEUMONICS for the instruction type, and operands BUT, now we need something to translate assembly lang. to machine lang.: an ASSEMBLER lastly, if we had a program that translated HLL programs to assembly language, then we'd have it made. a COMPILER does this. complete picture: ----------- ------------ HLL ---> | compiler|---> assembly --->| assembler|--->machine ----------- language ------------ languagethis course deals with the software aspects of assembly language, assemblers and machine language. It also deals with the hardware aspects of what the computer does to execute programs. It is an introduction to study of COMPUTER ARCHITECTURE: the interface between hardware and software.about COMPUTER ARCHITECTURE the relation between hardware (stuff you can touch) and software (programs, code) I can design a computer that has hardware which executes programs in any programming language. For example, a computer that directly executes Pascal. So, why don't we do just that? 1. From experience (in the engineering community), we know that the hardware that executes HLL programs directly are slower than those that execute a more simple, basic set of instructions. 2. Usability of the machine. Not everyone wants a Pascal machine. ANY high level language can be translated into assembly language. In this class, in whatever language you are writing programs,it will look like you have a machine that executesthose programs directly.What we will do: hll ---> SAL ---> MAL ---> TALwe assume that you know a HLL. From that, we can give youSAL. Later in the semester, you will learn MAL and TAL.HLL, SAL, and MAL are each abstractions.Each defines a computer architecture.TAL happens to be a real (manufactured) architecture. basic computer operation------------------------simplified diagram of a computer system (hardware!) ------- ---------- | CPU | <---------> | memory | ------- | ---------- | | ------- | I/O | -------CPU -- controls the running of programs executes instructions makes requests of the memorymemory -- where programs and program variables are stored handles requests from the CPU (STORED PROGRAM COMPUTER concept)interaction between CPU and memory. to execute an instruction, the CPU must be able to request 3 things from memory: 1. instruction FETCH 2. operand (variable) load LOAD 3. operand (variable) store STORE the memory really only needs to be able to do 2 operations 1. read (fetch or load) 2. write (store) where? a label specifies a unique place in memory. a label is often identified as an address. read: CPU specifies an address and a read operation memory responds with the contents of the address write: CPU specifies an address, data to be stored, and a write operation memory responds by overwriting the data at the address specified for discussion: how (most) computers operate WRT the execution of instructions. discussion by a generic assembly language instruction example: mult a, b, c instructions and operands are stored in memory. before they can be used by the CPU, they must be fetched/loaded CPU steps involved: 1. fetch the instruction questions for later: which instruction, what address 2. figure out what the instruction is -- DECODE IT IS A MULT INSTRUCTION this also reveals how many operands there are, since the number of operands is fixed for any given instruction THERE ARE 3 OPERANDS 3. load operand(s) OPERANDS ARE b AND c 4. do the operation specified by the instruction MULTIPLY b AND c TOGETHER 5. store result(s) (if any) RESULT GOES INTO VARIABLE a next step: suppose we want to execute multiple instructions, like a program except for branch instructions, execute instructions sequentially the CPU must keep track of which instruction is to be executed it does this by the use of an extra variable contained within and maintained by the CPU, called a PROGRAM COUNTER, or PC the contents of the variable is the address of the next instruction to be executed. modify the above CPU steps: 1. fetch the instruction at the address given by the PC added step. modify the PC such that it contains the address of the next instruction to execute 2-5. the same as above The added step could come at any time after step 1. It is convenient to think of it as step 2. This set of steps works fine for all instructions EXCEPT branch instructions. branch instructions: example: beq x, y, label 1. fetch instruction -- address given by PC 2. update PC 3. decode (its a BEQ instruction) 4. fetch operands (x and y) 5. compare operands (for equality) 6. if equal, overwrite PC with address implied by 3rd operand (label)The CPU steps involved: 1. fetch the instruction 2. update PC 3. decode 4. load operand(s) 5. do the operation specified by the instruction 6. store result(s) (if any)notice that this series of steps gets repeated constantly --to make the computer useful, all that is needed is a wayto give the PC an initial value (the first instruction of a program),and to have a way of knowing when the program is done, so thePC can be given the starting address of another program.the cycle of steps is very important -- it forms the basis forunderstanding how a computer operates. The cycle of stepsis termed the INSTRUCTION FETCH/EXECUTE CYCLE.</pre>⌨️ 快捷键说明
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