OSWORD

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Miscellaneous actions with a control block

Specification

 6502   Z80   6809   PDP11   80x86   32016   ARM  On entry: On exit:
A A A R0 AL R1 R0 = function code   undefined
XY HL X R1 BX R2 R1 =>control block   undefined, control block updated
Control block for calls>&7F
&00 Length of sent control block
&01 Length of received control block
&02... Any other parameters

OSWORD &00 is the only call that is allowed to return data in registers. All other calls return data in the control block.

OSWORD calls with numbers &15-&7F must only have a maximum of 16 bytes in the control block. OSWORD calls with numbers greater than &7F must contain the number of parameters to send in the first byte of the parameter block, and the number of parameters to receive in the second byte of the parameter block, inclusive of the first two bytes. There are some calls that break this and will not work in all cases.

While it is not enforced by the API, the convention for high-numbered OSWORDs is for the control block contents to be:

  • XY?0: length of sent block (required)
  • XY?1: length of received block (required)
  • XY?2: action
  • XY?3: result, set to zero when calling
  • XY!4 onwards: parameter data

Then calls are made with code along the lines of:

  • !XY%=&2008:XY%?2=6:XY%!4=data:CALL OSWORD:result=XY%?3

OSWORD calls with numbers &E0 to &FF are "available for use by the user" and are passed directly to USERV on the 6502 I/O processor.[1] [2] This makes them easier to implement as only USERV needs to be hooked into instead of WORDV.

Calling from BBC BASIC

Entry points

  • BBC BASIC Entry Address: &FFF1
  • 6502 Entry Address: &FFF1, vectors via &020C
  • Z80 Entry Address: &FFF1, vectors via &FFF2
  • 6809 Entry Address: &FFF1, vectors via &FFF2
  • 80x86 Entry Address: INT &4A, vectors via &0000:0128
  • 32000 Entry Address: SVC &07
  • PDP-11 Entry Address: EMT 3, vector &03
  • ARM Entry Address: SWI &07 "OS_Word", vector &07

See Also

References

  1. R.I.M. Sadek (1986), The complete disc manual for the BBC Microcomputer, Macmillan Education Ltd. p.107.
  2. Acorn Computers (June 1992), Application note 004: Tube application note, p.5.

Jgharston 17:03, 6 November 2009 (UTC)