Editing Simplified Instructional Computer (section)

== SIC Architecture ==
The SIC machine has basic addressing, storing most memory addresses in hexadecimal integer format. Similar to most modern computing systems, the SIC architecture stores all data in binary and uses the [[two's complement]] to represent negative values at the machine level. Memory storage in SIC consists of 8-bit bytes, and all memory addresses in SIC are byte addresses. Any three consecutive bytes form a 24-bit 'word' value, addressed by the location of the lowest numbered byte in the word value. Numeric values are stored as word values, and character values use the 8-bit [[ASCII]] system. The SIC machine does not support floating-point hardware and has at most 32,768 bytes of memory. There is also a more complicated machine built on top of SIC called the Simplified Instruction Computer with Extra Equipment (SIC/XE). The XE expansion of SIC adds a 48-bit floating point data type, an additional memory addressing mode, and extra memory (1 megabyte instead of 32,768 bytes) to the original machine. All SIC assembly code is upwards compatible with SIC/XE.

SIC machines have several registers, each 24 bits long and having both a numeric and character representation:
:* '''A (0)''': Used for basic arithmetic operations; known as the accumulator register.
:* '''X (1)''': Stores and calculates addresses; known as the index register.
:* '''L (2)''': Used for jumping to specific memory addresses and storing return addresses; known as the linkage register.
:* '''PC (8)''': Contains the address of the next instruction to execute; known as the program counter register.
:* '''SW (9)''': Contains a variety of information, such as carry or overflow flags; known as the status word register.

In addition to the standard SIC registers, there are also four additional general-purpose registers specific to the SIC/XE machine:
:* '''B (3)''': Used for addressing; known as the base register.
:* '''S (4)''': No special use, general purpose register.
:* '''T (5)''': No special use, general purpose register.
:* '''F (6)''': Floating point accumulator register (This register is 48-bits instead of 24).

These five/nine registers allow the SIC or SIC/XE machine to perform most simple tasks in a customized assembly language. In the System Software book, this is used with a theoretical series of operation codes to aid in understanding the assemblers and linker-loaders required for the execution of [[assembly language]] code.