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OSBYTE &97 (151) - Write SHEILA

   X=offset within page &FE and Y=byte to write

Acorn MOS 1.20 writes the byte using STA &FE00,X. As it is an indexed-addressing instruction, the 6502 CPU always[1][2] 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.

A safer alternative is to call OSWORD &06 with !XY=&FFFFFE00 + offset, XY?4=byte.

See Also

Jgharston 22:08, 26 May 2009 (UTC)


  1. MOS Technology, Inc. (January 1976), MCS6500 Microcomputer Family Hardware Manual, appendix A, section A.3.4, p.A-6 (PDF p.173)
  2. Jarkko Sonninen et al. (1994), 64doc