SYSTEM SOFTWARE

BY B.R.MOHAN

ASSOCIATE PROF, CSESRINIVAS SCHOOL OF ENGINEERING, MUKKA

Introduction – This book introduces to the design and implementation of System Software

System Software – consists of a variety of

programs that support the operation of Computer.

System Software makes it possible for user to

focus on application / other problem to be solved

Without needing to know the internal details of

m/c working.

• When u first started a programming course, u may have used different types of System Software.

• Programs were written in HLL like C++ / Pascal, using a text-editor to create and modify the program.

• Programs were translated into m/c language using a compiler.

• Resulting m/c language was loaded into memory and prepared for execution by a loader / linker.

• Debugger is used to detect errors in the program.

• Later, programs were written in assembly language using macro instructions in these programs to read and write data / to perform other higher level functions.

• Assembler may be used that included macroprocessor to translate these programs into m/c language.

• Translated programs were prepared for execution by the loader / linker and may have been tested using the debugger.

• All these processes were controlled by user interacting with OS like UNIX / DOS typing commands at a keyboard. WIndows had menu driven commands.

• In either case, OS took care of m/c details for you irrespective of the fact that your m/c is connected to n/w or shared by other users

• Topics - Assemblers, Loaders and Linkers , Macroprocessors, Compilers and operating systems

System Software & Machine Architecture

System Software Application Software

Machine dependent Machine Independent

Support the operation and use of computing system

Primarily Concerned with solution of some problem using the computer as a tool

Focus is on the computing system, relating to architecture on which they run

Focus is on application

Eg – Assemblers, compilers, Operating system

Eg – Adobe , Software, MSWord,etc.

• System software are intended to support the operation and use of computing.

Hence they are usually related to architecture of the machine on which they to run.

Eg – Assemblers translate mnemonic instructions into machine code; the instruction formats , addressing modes etc. are of direct concern in assembler design.

• Similarly, compilers must generate m/c language code taking into account h/w characteristics like number & type of registers and m/c instructions available.

• Operating systems are directly concerned with management of all resources of a computing system. These are some of examples of m/c dependency.

Some aspects of ss that dont directly depend upon the type of computing system being supported.

Eg – The general design and logic of an assembler is basically same on most computers.

Some of code optimization techniques used by compilers are independent of target m/c.

Process of linking together independently assembled subprograms that dont depend upon computer being used.

Simplified Instructional Computer (SIC) – describe SIC• m/c is designed to illustrates common h/w features and

concepts.

• SIC comes in 2 versions• Standard model• XE version (XE – Extra equipment / Extra expensive)

• 2 versions are designed to be upward compatible • upward compatible – object program for standard SIC m/c will

also execute properly on SIC/XE system.

SIC M/C Architecture

Memory – 8 bit bytes.

3 consecutive bytes – word(24 bits).

byte oriented addresses

Words are addressed by the location of their lowest numberedbyte.

SIC Memory size – 215 bytes

Registers – 5 registers of special use - each register is of 24 bits in length

Mnemonic Number Special use

A 0 Accumulator – used for alu operations

X 1 Index reg. – for addressing

L 2 Linkage reg. – Jsub instruction stores ret.

Address in this register

PC 8 Prg. Ctr – contains address of next instruction to be

fetched for execution

SW 9 Status reg. – contains a variety of info. Including

condition code (CC)

Data Formats

• Integers are stored as 24 bit binary numbers

• 2’s complement representation is used for negative values

• Characters are stored using their 8-bit ascii codes

Instruction Formats – All m/c instructions on standard version of SIC have 24 bit format

• X bit indicate indexed addressing mode

Opcode x address

8 1 15

Addressing modes - 2 addressing modes are available with setting X bit in instruction. Table describes how target address is calculated given in the instruction. (x) – indicate contents of register

Mode Indication Target address calculation

Direct X=0 TA=address

Indexed X=1 TA=address + (X)

Direct Addressing Mode – Eg – LDA TEN

0000 0000 0 001 0000 0000 0000

0 0 0001

x

Effective address (EA) = 1000

Content of address 1000 is loaded into

accumulator

opcode Ten

Indexed Addressing mode – Eg – STCH BUFFER , X

Effective Address (EA) = 1000 + [X]= 1000 + content of IR reg.

0101 0100 1 001 0000 0000 0000

5

X

5 0 0 04

OPCODE BUFFER

Instruction Set – sic give basic set of instructions to do simple tasks like loading and storing registers (LDA, LDX, STA, STX, etc.), integer arithmetic operations (ADD, SUB, MUL , DIV) involving accumulator & word in memory , with result left in memory

• COMP compares the value in reg. A with word in memory, setting a CC to indicate result (<, =, or >).

• Conditional jump instructions (JLT, JEQ, JGT) can test the setting of CC and jump accordingly.

• 2 are provided for subroutine linkage.

• JSUB jumps to Subroutine, placing return address in L;

• RSUB returns by jumping to address contained in register L

I/O – On Standard SIC , i/o are performed by transferring 1 byte at a time to / from rightmost 8 bits of register A

• Each device is assigned an 8 bit code.

• 3 instructions, each of which specifies the device code as operand.

• TD – test device instruction tests whether addressed device is ready to send / receive a byte of data.

• Condition Code is sets to indicate result of this test

• A setting of < means device is ready to send to / receive from and = means device is not ready.

• A program needing to transfer data must wait until device is ready, then execute (RD) a readdata and (WD) writedata. This sequence is repeated for each byte of data to be read / written.

SIC / XE architecture

• Memory of structure SIC / XE is same as described for SIC.

• Maximum memory available on SIC / XE system – 220 = 1 MB

• This increase leads to change in addressing modes.

• Additional registers in SIC / XE

Mnemonic Number Special Use

B 3 Base register – used for addressing

S 4 General Working Registers – no

special registers

T 5 General Working Registers – no

special registers

F 6 Floating point Accumulator(48 bits)

Data Formats – SIC / XE provides the same data formats as standard version. There is a floating point data type with following format :

S exponent fraction

1 11

Fraction is a value from 0 to 1 i.e. assumed binary point is immediately before higher bit. For normalized floating point numbers , the higher order bit of fraction must be 1. Exponent is interpreted as unsigned binary number between 0 and 2047. If exponent has value e, and fraction has value e and the fraction has value f, the absolute value of number is represented as

f*2(e-1024)

Sign of floating point number is indicated by value of S(0 = +ve , 1 = -ve)

36

Format 1 (1 byte) –

• Eg – RSUB (Return to subroutine)

• Format 2 (2 bytes) –

• Eg – COMPR A,S (compare [A] & S)

opcode

8

0100 1100

4 C

opcode r1 r2

4 4 8

1010 0000 1000 0100

4-bit 4-bit 8-bit

A S

• Format 3 (3 bytes) –

• Eg – LDA #3 ( Load 3 to A)

• Format 4 (4 bytes) –

• Eg – +JSUB RDREC ( jump to address , 1036)

opcode n i x b p e displacement

0000 00 n i x b p e 0000 0000 0011

12 bit 6 bit

6 bit 12 bit

opcode n i x b p e address

6 bit 20 bit

0100 10 1 1 0 0 0 1 0000 0001 0000 0011 0110

4 10 0 63

1 1 1 1 1 1

Addressing Modes – 2 relative addressing modes are used for instructions assembled using Format 3

Mode Indication Target address calculation

Base relative B=1, p=0 T.A = (B) + disp (0 << disp << 4095)

Program-counter relative B=0, p=1 T.A = (PC) + disp (-2048 << disp << 2047)

For base-relative addressing , displacement field disp in format 3 instruction is interpreted as a 12-bit unsigned integer.

1056 STX LENGTH

EA = LENGTH = 0033 EA = disp+[B][B]=0033Disp=0 The [033] i.e. content of address 0033 is loaded to index register x.For PC relative addressing , this field is interpreted as a 12 bit signed

integer, with negative values represented in 2’s complement notation.

0000 STL RETADR

RETADR =0330PC = 0003(address of next instruction)Disp =002DLinkage register contains the contents of RETADR 0030

0001 00 1 1 0 1 0 0 0000 0000 0000

1206

0001 01 1 1 0 0 1 0 0000 0010 1101

1 27 2 0 D

Object code

ea=retadr = 30 Ea= disp+pc Ea = 2d+0003

• Linkage register contain [retadr] = 0030

• If b & p = 0 disp field in format 3 is taken to be target address