1. SYSTEM SOFTWARE BY B.R.MOHAN ASSOCIATE PROF, CSE SRINIVAS SCHOOL OF ENGINEERING, MUKKA

2. 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.

3. 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.

4. 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.

5. 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

6. System Software & Machine Architecture
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.

7. 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.

8. 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.

9. 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.

10. 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

11. Registers – 5 registers of special use - each register is of 24 bits in length
Mnemonic
Number
Special use A
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)

12. 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

13. X bit indicate indexed addressing mode Opcode x address
Instruction Formats – All m/c instructions on standard version of SIC have 24 bit format
X bit indicate indexed addressing mode
Opcode x address 15 8 1

14. 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)

15. Direct Addressing Mode – Eg – LDA TEN
Effective address (EA) = 1000
Content of address 1000 is loaded into
accumulator 1
opcode x
Ten

16. Indexed Addressing mode – Eg – STCH BUFFER , X
Effective Address (EA) = [X] = content of IR reg. 5 4 5
OPCODE X
BUFFER

17. 2 are provided for subroutine linkage.
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

18. 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

19. 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.

20. 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

21. Base register – used for addressing
Mnemonic
Number
Special Use B 3
Base register – used for addressing S 4
General Working Registers – no special registers T 5 F 6
Floating point Accumulator(48 bits)

22. 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 36
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 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)

23. Eg – RSUB (Return to subroutine)
Format 1 (1 byte) – 8
opcode
Eg – RSUB (Return to subroutine)
Format 2 (2 bytes) –
Eg – COMPR A,S (compare [A] & S) 4 C
opcode
r r2 4 8 4 A S
8-bit
4-bit
4-bit

24. Eg – +JSUB RDREC ( jump to address , 1036)
Format 3 (3 bytes) –
Eg – LDA #3 ( Load 3 to A)
Format 4 (4 bytes) –
Eg – +JSUB RDREC ( jump to address , 1036) 1 1 1 1 1 1
opcode
n i x b p e
displacement
6 bit
12 bit
n i x b p e
6 bit
12 bit
opcode
n i x b p e
address
20 bit
6 bit 4 1 3 6

25. Target address calculation
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.

26. 1056 STX LENGTH 12 06
EA = LENGTH = 0033 EA = disp+[B] [B]=0033 Disp=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 STL RETADR RETADR =0330 PC = 0003(address of next instruction) Disp =002D Linkage register contains the contents of RETADR 0030 1 7 2 2 D
Object code

27. 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