Simplified Instructional Computer (SIC) Notes | Study Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE)

Introduction

Simplified Instructional Computer (SIC) is a hypothetical computer that has hardware features which are often found in real machines. There are two versions of this machine:

1. SIC standard Model
2. SIC/XE(extra equipment or expensive)

Object program for SIC can be properly executed on SIX/XE which is known as upward compatability.

SIC Machine Architecture/Components –

1. Memory

• Memory is byte addressable that is words are addressed by location of their lowest numbered byte.
• There are 2¹⁵ bytes in computer memory (1 byte = 8 bits)
  3 consecutive byte = 1 word (24 bits = 1 word)

2. Registers: There are 5 registers in SIC. Every register has an address associated with it known as register number. Size of each register is 3 bytes. On basis of register size, integer size is dependent.
(i) A(Accumulator-0): It is used for mathematical operations.
(ii) X(Index Register-1): It is used for addressing.
(iii) L(Linkage Register-2): It stores the return address of instruction in case of subroutines.
(iv) PC(Program Counter-8): It holds the address of next instruction to be executed.(v) SW(Status Word-9): It contains the variety of information
Status Word Register:

• mode bit refers to user mode(value=0) or supervising mode(value=1). It occupies 1 bit.[0]
• state bit refers whether process is in running state(value=0) or idle state(value=1). It also occupies 1 bit.[1]
• id bit refers to process id(PID). It occupies 3 bits.[2-5]
• CC bit refers to condition code i.e. It tells whether device is ready or not. It occupies 2 bits.[6-7]
• Mask bit refers to interrupt mask. It occupies 4 bits.[8-11]
• X refers to unused bit. It also occupies 4 bits.[12-15]
• ICode refers to interrupt code i.e. Interrupt Service Routine. It occupies the remaining bits.[16-23]

3. Data Format

• Integers are represented by 24 bit.
• Negative numbers are represented in 2’s complement.
• Characters are represented by 8 bit ASCII value.
• No floating point representation is available.

4. Instruction Format

All instructions in SIC have 24 bit format.

• If x=0 it means direct addressing mode.
• If x=1 it means indexed addressing mode.

5. Instruction Set

• Load And Store Instructions: To move or store data from accumulator to memory or vice-versa. For example LDA, STA, LDX, STX etc.
• Comparison Instructions: Used to compare data in memory by contents in accumulator. For example COMP data.
• Arithmetic Instructions: Used to perform operations on accumulator and memory and store result in accumulator. For example ADD, SUB, MUL, DIV etc.
• Conditional Jump: compare the contents of accumulator and memory and performs task based on conditions. For example JLT, JEQ, JGT
• Subroutine Linkage: Instructions related to subroutines. For example JSUB, RSUB

6. Input and Output: It is performed by transferring 1 byte at a time from or to rightmost 8 bits of accumulator. Each device has 8 bit unique code.

There are 3 I/O instructions:

• Test Device (TD) tests whether device is ready or not. Condition code in Status Word Register is used for this purpose. If cc is < then device is ready otherwise device is busy.
• Read data(RD) reads a byte from device and stores in register A.
• Write data(WD) writes a byte from register A to the device.