An Introduction to the SIC architecture

SIC’s Feature Set

Specifications

• 8-bit bytes (1 byte = 8 bits)
• 3-byte words (3 bytes = 1 word)
• 32 kilobytes of memory (2¹⁵ bytes)
• 24-bit unsigned integers with two’s complement for negation (no floating points)
• 8-bit ASCII for characters
• Up to 265 “devices” to write bytes to or read bytes from

Registers

SIC has 5 registers (listed as mnemonic (number):):

• A (0): accumulator (used for arithmetic operations)
• X (1): index (used for storing and calculating addresses)
• L (2): linkage (used for jumping to memory addresses and return values)
• PC (8): program counter (used for holding the address of the next instruction)
• SW (9): status word (holds various info, e.g. flags)

Each register has its own special purpose and all 5 registers are 24 bits in size (3 words).

The A register

The A register is used when doing integer arithmetic operations, where its value is modified (added to, subtracted from etc.) with the value from the address, specified in the instruction. The result is always stored back into register A.

The X register

The X register is used for storing an offset, which is used when indexed addressing is enabled (explained in the next section).

The L register

The L register is used for storing the return address, when going into a subroutine.

The PC register

The PC register is used for storing the next instruction’s address, so that the microprocessor know where to find the instruction to execute. The address stored in the register is implicitly (automatically) incremented after executing each instruction. The value of the PC register doesn’t normally need to be set manually, so SIC doesn’t offer a separate instruction for it.

The SW register

The SW register is split into multiple meaningful sections of bits, which each hold useful information:

• Bit 0: Mode
  • If mode = 0 then SIC is in user mode
  • If mode = 1 then SIC is in supervising mode
• Bit 1: State
  • If state = 0 then SIC is in running state
  • If state = 1 then SIC is in idle mode
• Bits 2-5: PID (the process ID)
• Bits 6-7: CC (Condition Code - set by the compare operation and used in conditional jumps)
• Bits 8-11: Interrupt mask
• Bits 12-15: Unused
• Bits 16-23: Interrupt Code (Interrupt Service Routine)

Bits	0	1	2-5	6-7	8-11	12-15	16-23
Length	1 bit	1 bit	4 bits	2 bits	4 bits	4 bits	8 bits
Purpose	Mode	State	PID	CC	Interrupt mask	Unused	Interrupt Code

TODO: Figure out what separate sections do

The value of the SW register doesn’t normally need to be set manually, so SIC doesn’t offer a separate instruction for it.

Instruction Format

SIC only supports a single instruction format (24-bit):

• 8 bits for opcode
• 1 bit for indexing flag (x bit)
• 15 bits for the address

Bits	0-7	8	9-23
Length	8 bits	1 bit	15 bits
Purpose	Opcode	Indexing flag	Address

The microprocessor has to know where to get the value to use in an operation. It can find the memory address of that value (the target address or TA) from the address bits, but by combining the addressing bits with the indexing bit value, it can do this in two ways:

• If x = 0 then TA equals the address directly (direct addressing mode)
• If x = 1 then TA equals the address plus the value of the X register (indexed addressing mode)

x bit	Target address	Addressing mode
0	TA = address	Direct
1	TA = address + (X)	Indexed

(X) means the value in register X.

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