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Separate from the stack definition of a MISC architecture, is the MISC architecture being defined by the number of instructions supported. Typically a minimal instruction set computer is viewed as having 32 or fewer instructions, [1] [2] [3] where NOP, RESET, and CPUID type instructions are usually not counted by consensus due to their fundamental nature.
The first was the CISC (Complex Instruction Set Computer), which had many different instructions. In the 1970s, however, places like IBM did research and found that many instructions in the set could be eliminated. The result was the RISC (Reduced Instruction Set Computer), an architecture that uses a smaller set of instructions.
The instruction set architecture (ISA) that the computer final version (SAP-3) is designed to implement is patterned after and upward compatible with the ISA of the Intel 8080/8085 microprocessor family. Therefore, the instructions implemented in the three SAP computer variations are, in each case, a subset of the 8080/8085 instructions. [2]
Machine instructions created in earlier microprocessors were retained throughout microprocessor upgrades. This enabled consumers to purchase new computers without having to purchase new application software. The major categories of instructions are: [c] Memory instructions to set and access numbers and strings in random-access memory.
The IBM 704, 709, 704x and 709x store one instruction in each instruction word; IBM numbers the bit from the left as S, 1, ..., 35. Most instructions have one of two formats: Generic S,1-11 12-13 Flag, ignored in some instructions 14-17 unused 18-20 Tag 21-35 Y Index register control, other than TSX S,1-2 Opcode 3-17 Decrement 18-20 Tag 21-35 Y
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decrement (DJN, Decrement and branch (Jump) if Nonzero) [7] increment (P1eq, Plus 1 and branch if equal to another value) [8] subtraction (subleq, subtract and branch if less than or equal to zero) [9] [10] positive subtraction when possible, else branch (Arithmetic machine) [11]
The instruction set for the floating point coprocessor also had several instructions added to it. An IEEE 754-compliant floating-point square root instruction was added. It supported both single- and double-precision operands. A set of instructions that converted single- and double-precision floating-point numbers to 32-bit words were added.