The Instruction Set Architecture

Hardware – Software boundary Java Program C Program Ada Program Compiler Instruction Set Architecture Microcode Hardware

Data types 4 The ISA defines the data types native to a particular machine. Some examples of numeric data types that may or may not be present on the machine: –Signed integer –Unsigned integer –Floating point –Binary Coded Decimal (BCD) integers –Packed numeric representations 4 Java does not support unsigned, BCD, or packed integer representations.

Instruction Format 4 The ISA defines the instruction format(s) for the machine –Fixed or variable length instructions –0, 1, 2, or 3 operand instructions –Addressing modes 4 The Java Virtual Machine –9 instruction formats –Variable length –Little in the way of addressing modes; immediate, indexed

Examples 4 See handout of IBM System 360 ISA –5 instruction formats –A long list of instructions –Condition codes 4 This is a CISC (Complex Instruction Set Computer) Architecture

Addressing modes 4 Why more than one type? –Short is good, but not sufficient for all purposes –Immediate address – the data is in the same memory word as the instruction –Register – the data is in one of the registers, memory locations on the processor chip itself –Absolute – specify the exact memory location in use –Indexed – Data is specified by a base and displacement –Indirect – address specified contains an address, rather than data

ISA and Microarchitecture 4 So, you have this wonderful idea for improving performance of your processor –Will all the existing software still run? –Will the same operating system work? 4 The microarchitecture separates the actual hardware from the ISA. –Do the hardware improvements. –Modify the microcode to match the existing ISA to the new hardware. All existing applications continue to run. New applications can take advantage of new features.

Language Levels Hardware: understands voltages Microarchitecture: bit settings that activate specific data flows ISA: instruction set, addressing modes of a specific line of computers C, Java, etc.: designed for representing a problem solution; independent of the machine Higher level images of the computer: Spreadsheets, Word processors, databases – closer to the problem

Access to the hardware 4 Programs that can manipulate the hardware can be dangerous. 4 Hardware is protected by providing several classes of access rights. 4 Operating system has complete access 4 User programs are limited. 4 Users can access the hardware by invoking operating system functions.

Why many high level languages? 4 Notice the position of the high level languages in our chart –They exist to make it easier to express the problem –Different application areas have different needs in terms of expressing problems. 4 Ex. APL – single instruction to multiply two matrices

Compiler 4 A compiler is a program 4 Its input is the text of a program in a particular language, say c 4 The compiler translates the c code into the instruction set understood by the computer 4 The translation is not a line by line transliteration. Optimization.

Instructions and Data 4 Two distinct kinds of things –The machine understands a finite subset of possible bit patterns as instructions. –Data can take any bit pattern. 4 Usually stored separately. Data accessed when an instruction calls for it.

Black boxes 4 To reduce complexity, we sometimes show only the input and output of an operation without the details of how it gets done. We call it a black box, because the content of the box is hidden. a, b a + b Turing machine that adds We can combine the boxes and use them any way we want.

Turing Machines 4 Mathematical model of a machine 4 Turing was interested in understanding what could be computed and what could not be computed. 4 The general purpose computers of today are implementations of the Turing model 4 That’s why all computers can do the same things, given enough time and memory – because they are all Turing machines.