Sq Usage Notes - Hitachi SH7750 Hardware Manual

4.7.6

SQ Usage Notes

If an exception occurs within the three instructions preceding an instruction that writes to an SQ in
the SH7750 and SH7750S, a branch may be made to the exception handling routine after
execution of the SQ write that should be suppressed when an exception occurs.
This may be due to the bug described in (1) or (2) below.
(1) When SQ data is transferred to external memory within a normal program
If a PREF instruction for transfer from an SQ to external memory is included in the three
instructions preceding an SQ store instruction, the SQ is updated because the SQ write that
should be suppressed when a branch is made to the exception handling routine is executed, and
after returning from the exception handling routine the execution order of the PREF instruction
and SQ store instruction is reversed, so that erroneous data may be transferred to external
memory.
(2) When SQ data is transferred to external memory in an exception handling routine
If store queue contents are transferred to external memory within an exception handling
routine, erroneous data may be transferred to external memory.
Example 1: When an SQ store instruction is executed after a PREF instruction for transfer from that same SQ
to external memory
PREF instruction
; PREF instruction for transfer from SQ to external memory
; Address of this instruction is saved to SPC when exception occurs.
; Instruction 1, instruction 2, or instruction 3 may be executed on return from exception handling routine.
Instruction 1 ; May be executed if an SQ store instruction.
Instruction 2 ; May be executed if an SQ store instruction.
Instruction 3 ; May be executed if an SQ store instruction.
Instruction 4 ; Not executed even if an SQ store instruction.
Example 2: When an instruction that generates an exception branches using a branch instruction
Instruction 1 (branch instruction) ; Address of this instruction is saved to SPC when exception occurs.
Instruction 2 ; May be executed if instruction 1 is a delay slot instruction and an instruction to store data to SQ.
Instruction 3
Instruction 4
Instruction 5
Instruction 6
Instruction 7 (branch destination of instruction 1)
; May be executed if an SQ access instruction.
Instruction 8 ; May be executed if an SQ store instruction.
Rev. 6.0, 07/02, page 125 of 986