Simplified Instructional Computer (SIC) Architecture: A Comprehensive Guide
Explore the architecture of the Simplified Instructional Computer (SIC), a foundational model for understanding computer design principles. This guide details SIC's memory organization, registers, instruction formats, and addressing modes, providing a clear understanding of basic computer architecture.
Simplified Instructional Computer (SIC) Architecture
The Simplified Instructional Computer (SIC) is a hypothetical computer architecture used to illustrate fundamental computer design principles. It serves as a simplified model to understand various hardware concepts and features. SIC has two versions: the standard SIC and the enhanced SIC/XE.
SIC Standard Model: Architecture and Components
Memory Organization
SIC's memory is organized as a sequence of 8-bit bytes. A word consists of 3 consecutive bytes (24 bits). The address of a word is the address of its lowest-numbered byte. The total memory size is 215 bytes.
Registers
SIC uses several registers, each with a specific purpose:
| Register | Mnemonic | Size (bytes) | Purpose |
|---|---|---|---|
| Accumulator | A | 3 | Arithmetic operations |
| Index Register | X | 3 | Addressing |
| Linkage Register | L | 3 | Stores return address for subroutines |
| Program Counter | PC | 3 | Holds address of the next instruction |
| Status Word | SW | 3 | Contains condition codes and other status information |
Status Word (SW) Details
The status word (SW) is a 24-bit register containing various status flags:
| Bits | Field | Description |
|---|---|---|
| [0] | Mode | 0: User mode, 1: Supervisor mode |
| [1] | State | 0: Running, 1: Idle |
| [2-5] | ID | Process ID |
| [6-7] | CC | Condition Code (e.g., less than, equal to, greater than) |
| [8-11] | Mask | Interrupt mask |
| [12-15] | X | Unused bits |
| [16-23] | ICode | Interrupt code |
Data Formats
- Integers: 24 bits
- Characters: 8-bit ASCII
- Floating-point numbers: Not supported (supported in SIC/XE)
Instruction Format
All SIC instructions use a 24-bit format.
(A diagram illustrating the SIC instruction format would be beneficial here. It should show the opcode, the address field, and the index register bit (X). The use of X=0 for direct addressing and X=1 for indexed addressing should be explicitly indicated in the diagram.)
SIC Instruction Set
The SIC instruction set includes arithmetic, load/store, comparison, subroutine linkage, conditional jump, and I/O instructions.
(Descriptions of the arithmetic, load and store, comparison, subroutine linkage, conditional jump, and I/O instructions, along with examples are given in the original text and should be included here.)
SIC/XE Enhancements
SIC/XE is an enhanced version of SIC, adding features such as:
- Larger address space (1 MB or 220 bytes).
- Additional addressing modes.
- Support for floating-point numbers.
- Interrupts and memory protection.
SIC/XE Memory Organization
SIC/XE uses 8-bit bytes and 24-bit words, but it has a larger memory space than the standard SIC. Addressing in SIC/XE is always byte-addressable.
SIC/XE Registers
(The description of registers in SIC/XE should be given here. A comparison with the standard SIC registers would be helpful.)
Conclusion
The SIC architecture, particularly the SIC/XE enhancement, serves as a simplified but valuable model for understanding fundamental computer architecture concepts.
Input/Output (I/O) Operations in SIC/XE
I/O Channels in SIC/XE
The SIC/XE (Simplified Instructional Computer - Extended) architecture includes an I/O channel, enabling the CPU to perform I/O operations concurrently with other processing tasks. This overlapping of I/O and computation significantly improves system efficiency.
I/O Instructions in SIC/XE
Three instructions control the I/O channel:
SIO(Start I/O): Initiates an I/O operation.TIO(Test I/O): Checks the status of an I/O device.HIO(Halt I/O): Stops an I/O operation.
Each I/O device is assigned an 8-bit address. Data is transferred in bytes to or from the rightmost byte of register A. (For the specific instruction formats and usage of these instructions, refer to the instruction set documentation of SIC/XE.)
Example: I/O Operations in SIC/XE
(An example illustrating the use of TD (Test Device), JEQ (Jump if Equal), RD (Read Data), and STCH (Store Character) instructions for input operations and TD, JEQ, LDCH (Load Character), and WD (Write Data) instructions for output operations should be added here. The code should show how these instructions interact to handle input and output operations concurrently with CPU processing. The original text's example code would be included here.)
Conclusion
The I/O channel in SIC/XE allows for efficient overlapping of I/O operations with processing, improving overall system performance. This approach is a significant advancement over simpler systems that require the CPU to directly handle all I/O tasks.