PDP-11 Second Processor

An emulated Second Processor for the BBC/Master containing a DEC PDP-11.

The PDP-11 client MOS provides the basic system required to communicate with the host computer as well as a cut-down set of PDP-11 Unix functions.

Software and documentation for the PDP-11 CoProcessor are available at mdfs.net.

MOS API Calls

Programs communicate with the kernal and MOS via the EMT trap instruction. Parameters are passed in registers R0 onwards.

On entry, R0-R7 contain values to pass to EMT call
On exit,  R0-R7 contain returned data
          V=0 - no error occured
          V=1 - error occured, R0=>error block

EMT &00 - OS_QUIT

On entry:           On exit:
R0=Return value

EMT &01 - OS_CLI

On entry:           On exit:
R0=>command string  R0=Return value

EMT &02 - OS_BYTE

On entry:           On exit
R0=function         R0=preserved
R1=first parameter  R1=returned word
R2=second parameter R2=returned word DIV 256
                    Cy=returned carry flag

EMT &03 - OS_WORD

On entry:           On exit:
R0=function         Control block updated
R1=>control block   R1=returned line length for OSWORD 0

EMT &04 - OS_WRCH

On entry:           On exit:
R0=character        R0=preserved

EMT &05 - OS_NEWL

On entry:           On exit:
                    R1=13

EMT &06 - OS_RDCH

On entry:           On exit:
                    R0=character
                    Cy=carry flag

EMT &07 - OS_FILE

On entry:           On exit:
R0=function         R0=result
R1=>control block   R1=preserved

EMT &08 - OS_ARGS

On entry:           On exit:
R0=function         R0=result
R1=handle           R1=preserved
R2=>data            R2=data

EMT &09 - OS_BGET

On entry:           On exit:
                    R0=byte
R1=handle           R1=preserved
                    Cy=carry flag

EMT &0A - OS_BPUT

On entry:           On exit:
R0=byte             R0=preserved
R1=handle           R1=preserved

EMT &0B - OS_GBPB

On entry:           On exit:
R0=function
R1=>control block   R1=preserved
                    Cy=carry flag

EMT &0C - OS_FIND

On entry:           On exit:
R0=function         R0=handle
R1=>string or handle

EMT &0D - OS_SYST

On entry:
  R0=0 - Loads and enters BASIC
         R1=0 or R1=>parameters
  R0=1 - Reinitialises vectors, handlers and EMT dispatch table
  R0=2 - Reinitialises handlers and EMT dispatch table only
On exit:
  All preserved

EMT &0E - OS_HANDLER

On entry:                On exit:
R0=EMT or handler number R0=preserved
R1=handler or 0 to read  R1=previous handler
R2=buffer or 0 to read   R2=previous buffer

If R0>=0, the call redirects the specified EMT call, returning the previous handler

If R0<0, the call sets and reads the following environment handlers:
  R0  R1                  R2
  -1  Exit handler
  -2  Escape handler      Escape flag
  -3  Error handler       Error buffer
  -4  Event handler
  -5  Unknown IRQ handler
  -6  ignored             EMT dispatch table

EMT &0F - OS_ERROR

EMT &0F is followed by the error block in memory:
   EMT 15
   EQUB error
   EQUS "error string"
   EQUB 0

See also

• BBC BASIC (PDP11)

Jgharston 18:31, 11 September 2008 (BST)