Flag generation and use in processor with same processing for operation on small size operand as low order bits portion of operation on large size operand
View Patent ↗The RISC data processor is based on the idea that in case that there are many flag-generating instructions, the number of flags generated by each instruction is increased so that a decrease of flag-generating instructions exceeds an increase of flag-using instructions in quantity, thereby achieving the decrease in instructions. With the data processor, an instruction for generating flags according to operands' data sizes is defined. To an instruction set handled by the RISC data processor, an instruction capable of executing an operation on operand in more than one data size, which performs a process identical to an operation process conducted on the small-size operand on low-order bits of the large-size operand, and generates flags capable of coping with the respective data sizes regardless of the data size of each operand subjected to the operation is added. Thus, the reduction in instruction code space of the RISC data processor tight in instruction code space can be achieved.
1. A data processor of Reduced Instruction Set Computer type, comprising:
an instruction execution unit; and
an instruction set having a first instruction which caused the instruction-execution unit to execute a process involving flag generation, and a second instruction which causes instruction-execution unit to execute a process involving use of a flag,
wherein the instruction-execution unit has an operation circuit operable to perform a process according to a result of decode of an instruction, a flag latch circuit, and a flag select circuit,
wherein the operation circuit is configured to execute an operation process on operands of large data size and of small data size according to a result of decode of the first instruction,
wherein the operation circuit is configured to perform a first process on the operand of a small data size, to generate a first flag group and a second flag group, and to select a first flag out of the first flag group and a second flag out of the second flag group according to a result of decode of the first instruction,
wherein the operation circuit is configured to perform a second process on the operand of a large data size, to generate the first flag group and the second flag group, and to select a first flag out of the first flag group and a second flag out of the second flag group according to a result of decode of the first instruction,
wherein the operation of the first process on the small data size operand is identical to the operation of the second process on the low-order bits of a large data size operand,
wherein the first flag group and the second flag group generated by the operation on the operand of the small data size and the first flag group and the second flag group generated by the operation on the operand of the large data size are the same flag groups,
wherein the flag latch circuit latches the first flag and the second flag generated by the operation circuit according to a result of decode of the first instruction, and
wherein the flag select circuit selects one of the first flag and the second flag latched by the flag latch circuit, according to a result of decode of the second instruction.
2. The data processor according to claim 1 ,
wherein the operation circuit generates flags of Signed Large and Signed Small, Unsigned Large and Unsigned Small, Zero, Overflow, Carry and Shift-out bit for each data size, and
wherein one of the flags thus generated is selected by the first instruction, and latched by the flag latch circuit for each operand size.
3. The data processor according to claim 2 ,
wherein the small data size is 32 bits and the large data size is 64 bits.