OSBYTE &B9

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OSBYTE &B9 (185) - Read/Write MOS copy of palette register/ROM polling semaphore

   On the BBC, B+ and Master, this contains the MOS's copy of the palette
   register in the video ULA.
   
   On the Electron there is no video ULA, and this location contains the ROM
   polling semaphore. When the semaphore is non-zero the 100Hz ROM service
   call &15 is sent to ROMs. The semaphore is normally accessed with *FX23
   to increment it and *FX24 to decrement it.

See Also

Jgharston 22:30, 26 May 2009 (UTC)