RISC-V Second Processor

(initial documentation)

An emulated Second Processor for the BBC/Master containing a RISC-V processor.

The RISC-V client MOS provides the basic system required to communicate with the host computer.

MOS API Calls

Programs communicate with the kernal and MOS via the ECALL instruction. Parameters are passed in registers A0 onwards.

On entry, A0-A7 contain values to pass to ECALL call
          A8=ECALL number
On exit,  A0-A7 contain returned data
          V=0 - no error occured
          V=1 - error occured, R0=>error block

ECALL &00 - OS_QUIT

On entry:           On exit:
A0=Return value

ECALL &01 - OS_CLI

On entry:           On exit:
A0=>command string  A0=Return value

ECALL &02 - OS_BYTE

On entry:           On exit
A0=function         A0=preserved
A1=first parameter  A1=returned word
A2=second parameter A2=returned word DIV 256
                    Cy=returned carry flag

ECALL &03 - OS_WORD

On entry:           On exit:
A0=function         Control block updated
A1=>control block   A1=returned line length for OSWORD 0

ECALL &04 - OS_WRCH

On entry:           On exit:
A0=character        A0=preserved

ECALL &05 - OS_NEWL

On entry:           On exit:
                    A0=13

ECALL &06 - OS_RDCH

On entry:           On exit:
                    A0=character
                    Cy=carry flag

ECALL &07 - OS_FILE

On entry:            On exit:
A0=function          A0=onject type
A1=>filename         A1=preserved
A2=load address      A2=load address
A3=exec address      A3=exec address
A4=start address     A4=length
A5=end address/attrs A5=attributes

ECALL &08 - OS_ARGS

On entry:           On exit:
A0=function         A0=result
A1=handle           A1=preserved
A2=data             A2=data

ECALL &09 - OS_BGET

On entry:           On exit:
                    A0=byte
A1=handle           A1=preserved
                    Cy=carry flag

ECALL &0A - OS_BPUT

On entry:           On exit:
A0=byte             A0=preserved
A1=handle           A1=preserved

ECALL &0B - OS_GBPB

On entry:           On exit:
A0=function         A0=result
A1=handle           A1=handle or cycle number
A2=start address    A2=updated address
A3=count            A3=updated count
A4=offset           A4=updated offset
                    Cy=carry flag

ECALL &0C - OS_FIND

On entry:           On exit:
A0=function         A0=handle
A1=>string or handle

ECALL &0D - OS_SYST

On entry:
  R0=subfunction

ECALL &0E - OS_HANDLER

On entry:                On exit:
A0=handler number

ECALL &0F - OS_ERROR

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

See also

• [[1]]

Jgharston (talk) 04:01, 6 September 2023 (CEST)