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Simplified Instructional Computer
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==Addressing Modes for SIC and SIC/XE== The Simplified Instruction Computer has three instruction formats, and the Extra Equipment add-on includes a fourth. The instruction formats provide a model for memory and data management. Each format has a different representation in memory: :*'''Format 1''': Consists of 8 bits of allocated memory to store instructions. :*'''Format 2''': Consists of 16 bits of allocated memory to store 8 bits of instructions and two 4-bits segments to store operands. :*'''Format 3''': Consists of 6 bits to store an instruction, 6 bits of flag values, and 12 bits of displacement. :*'''Format 4''': Only valid on SIC/XE machines, consists of the same elements as format 3, but instead of a 12-bit displacement, stores a 20-bit address. Both format 3 and format 4 have six-bit flag values in them, consisting of the following flag bits: :*'''n''': Indirect addressing flag :*'''i''': Immediate addressing flag :*'''x''': Indexed addressing flag :*'''b''': Base address-relative flag :*'''p''': Program counter-relative flag :*'''e''': Format 4 instruction flag
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