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Revision as of 00:21, 12 March 2021 by SteveF (talk | contribs) (Note that OSWRSC takes Y register into account)
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Writes a byte to screen.


On entry:
 ?&D6 = LSB of address to be read
 ?&D7 = MSB of address to be read
A = Byte to be written
On exit:
A,X,Y = preserved
C undefined

Stores a byte in an address the current display memory (in main or [[shadow RAM]]). The address to be written to is given in locations &D6 (low byte) and &D7 (high byte) plus Y, and byte to be written is passed in A. In other words, OSWRSC acts like STA (&D6),Y but accessing screen RAM (and affecting the flags). Although this call is the complement of OSRDSC which is able to read from sideways ROMs, this call only writes to the currently displayed memory and the currently selected sideways RAM bank.

Also note that the address is passed in a different location to OSRDSC.

Calling from BBC BASIC

  • BASIC does not call OSRDSC

Entry points

  • BBC BASIC Entry Address: none
  • 6502 Entry Address: &FFB3
  • Z80 Entry Address: none
  • 6809 Entry Address:
  • 80x86 Entry Address: none
  • 32000 Entry Address: none
  • PDP-11 Entry Address: none
  • ARM Entry Address: none


The call was introduced in MOS 2.00, and is absent in earlier versions. In MOS 2.00 calls write to the currently displayed screen memory. On the Master series the call writes to whichever screen memory is being accessed by the VDU.

OSWRSC is not implemented on non-6502 platforms and BBC BASIC does not emulate its entry point.


The function FNwm(), if used on the I/O processor, will write a byte to screen or main memory.


If the passed address is &FFFExxxx it writes to the currently displayed screen memory, otherwise it writes to main memory.

-- beardo 23:20, 5 October 2007 (BST) Jgharston (talk) 04:26, 29 March 2015 (UTC)