SH7101 datasheet(97/486 Pages) RENESAS

97 / 486 page

5. Exception Processing

Rev.2.00 Sep. 27, 2007 Page 63 of 448

REJ09B0394-0200

5.5.3

Illegal Slot Instructions

An instruction placed immediately after a delayed branch instruction is called “instruction placed

in a delay slot”. When the instruction placed in the delay slot is an undefined code, illegal slot

exception processing starts after the undefined code is decoded. Illegal slot exception processing

also starts when an instruction that rewrites the program counter (PC) is placed in a delay slot and

the instruction is decoded. The CPU handles an illegal slot instruction as follows:

1. The status register (SR) is saved to the stack.

2. The program counter (PC) is saved to the stack. The PC value saved is the target address of the

delayed branch instruction immediately before the undefined code or the instruction that

rewrites the PC.

3. The start address of the exception service routine is fetched from the exception processing

vector table that corresponds to the exception that occurred. That address is jumped to and the

program starts executing. The jump in this case is not a delayed branch.

5.5.4

General Illegal Instructions

When undefined code placed anywhere other than immediately after a delayed branch instruction

(i.e., in a delay slot) is decoded, general illegal instruction exception processing starts. The CPU

handles the general illegal instructions in the same procedures as in the illegal slot instructions.

Unlike processing of illegal slot instructions, however, the program counter value that is stacked is

the start address of the undefined code.

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