Simplified Instructional Computer (SIC)

A simplified Instructional Computer (SIC) is a hypothetical computer that has hardware features that 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 programs for SIC can be properly executed on SIC/XE which is known as upward compatibility. 

SIC Machine Architecture/Components –

1. Memory –

• Memory is byte-addressable that is words are addressed by the location of their lowest numbered byte.
• There are 2^15 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 a registration number. The 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 the instruction in case of subroutines. 
IV. PC(Program Counter-8): It holds the address of the next instruction to be executed. 
V. SW(Status Word-9): It contains a 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 the 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 bits.
• Negative numbers are represented in 2’s complement.
• Characters are represented by 8 bit ASCII values.
• No floating-point representation is available.

4. Instruction Format –
All instructions in SIC have a 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 results in the 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 the rightmost 8 bits of the accumulator. Each device has an 8-bit unique code. 
There are 3 I/O instructions: 

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

References:

Leland.L.Beck: An introduction to systems programming, 3rd Edition, Addison-Wesley, 1997.

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