system software

1.System Software and Machine Architecture

The Simplified Instructional Computer (SIC)
Traditional (CISC) Machines
Complex Instruction Set Computers
RISC Machines
Reduced Instruction Set Computers

2.System Software vs. Machine Architecture

Machine dependent
The most important characteristic in which most system software differ from application software
e.g. assembler translate mnemonic instructions into machine code
e.g. compilers must generate machine language code
Machine independent
There are aspects of system software that do not directly depend upon the type of computing system
e.g. general design and logic of an assembler
e.g. code optimization techniques

3.The Simplified Instructional Computer (SIC)

SIC is a hypothetical computer that includes the hardware features most often found on real machines
Two versions of SIC
standard model
extension version

4.SIC Machine Architecture (1/5)

Memory
215 bytes in the computer memory
3 consecutive bytes form a word
8-bit bytes
Registers

mnenomic number use
A 0 Accumulator
X 1 Index registers
L 2 linkage registers
PC 8 Program counter
SW 9 Status word

Data Formats
Integers are stored as 24-bit binary numbers; 2’s complement representation is used for negative values
No floating-point hardware

Instruction Set
load and store: LDA, LDX, STA, STX, etc.
integer arithmetic operations: ADD, SUB, MUL, DIV, etc.
All arithmetic operations involve register A and a word in memory, with the result being left in the register
comparison: COMP
COMP compares the value in register A with a word in memory, this instruction sets a condition code CC to indicate the result

Instruction Set
conditional jump instructions: JLT, JEQ, JGT
these instructions test the setting of CC and jump accordingly
subroutine linkage: JSUB, RSUB
JSUB jumps to the subroutine, placing the return address in register L
RSUB returns by jumping to the address contained in register L

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