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Instruction Classes
The R3900 Processor Core has five classes of CPU instructions, as follows.
• Load/store
These instructions transfer data between memory and general-purpose registers. "Base register + 16-bit
signed immediate offset" is the only supported addressing mode, so the format of all instructions in this
class is I-type.
• Computational
These instructions perform arithmetic logical and shift operations on register values. The format can be
R-type (when both operands and the result are register values) or I-type (when one operand is 16-bit
immediate data).
• Jump/branch
These instructions change the program flow. A jump is always made to a paged absolute address,
constructed by combining a 26-bit target address with the upper 4 bits of the program counter (J-type
format) or to a 32-bit register address (R-type format). In a branch instruction, the target address is the
program counter value plus a 16-bit offset. With a Jump And Link instruction, the return address is saved
in general register r31.
• Coprocessor
These instructions execute coprocessor operations. Coprocessor load and store instructions have the I-
type format. The format of coprocessor computational instructions differs from one coprocessor to
another.
• Special
These instructions support system calls and breakpoint functions. The format is always R-type.