OSBYTE &97 (151) - Write SHELIA
X=offset within page &FE and Y=byte to write
Acorn MOS 1.20 writes the byte using
STA &FE00,X (at address &EAFF). As it is an indexed-addressing instruction, the 6502 CPU performs a dummy read immediately before the write. Some hardware, though not all, may operate inappropriately due to the dummy read: outputs could momentarily be set to the wrong level, or an interrupt condition could be cleared and lost. Review the datasheet for the hardware being accessed to see whether it will be adversely affected.
The safe alternative is to call OSWORD &06 with !XY = &FFFFFExx and XY?4 = byte to write.
Jgharston 22:08, 26 May 2009 (UTC)