OSWORD &05

OSWORD &05 (5) - Read I/O processor memory (Acorn MOS 1.00 and later)

 On entry:
   XY!0=address to read from
 On exit:
   XY?4=the byte read

Some systems recognise screen memory at &FFFExxxx and sideways ROMs at &FFrr8000-&FFrrBFFF.

Coding

The following routines can be used to copy data to and from I/O memory regardless of the location of the calling program. It requires X%=>5-byte control block, Y%=X%DIV256.

 DEFPROCmem_rd(io%,mem%,num%)
 A%=5:REPEAT
   !X%=io%:CALL&FFF1:?mem%=X%?4
   io%=io%+1:mem%=mem%+1:num%=num%-1
 UNTILnum%<1:ENDPROC
 :
 DEFPROCmem_wr(io%,mem%,num%)
 A%=6:REPEAT
   !X%=io%:X%?4=?mem%:CALL&FFF1
   io%=io%+1:mem%=mem%+1:num%=num%-1
 UNTILnum%<1:ENDPROC
 
 REM To read a single byte:
 !X%=addr:A%=5:CALL&FFF1:byte=X%?4
 :
 REM To write a single byte
 X%?4=byte:!X%=addr:A%=6:CALL&FFF1

By copying code to the I/O processor and the I/O processor's USERV you can call an arbitary address in the I/O processor.

 DEFPROCmem_call(io%,A$):LOCAL old%
 !X%=&200:REPEAT
   A%=5:CALL&FFF1:old%=256*X%?4+old%DIV256
   A%=6:X%?4=io%:CALL&FFF1:io%=io%DIV256
   ?X%=?X%+1
 UNTIL ?X%=2
 OSCLI "LINE "+A$
 !X%=&200:REPEAT
   X%?4=old%:CALL&FFF1:old%=old%DIV256
   ?X%=?X%+1
 UNTIL ?X%=2
 =B%
 :
 DEFFNmem_call(io%,B%,C%):LOCAL old%
 !X%=&200:REPEAT
   A%=5:CALL&FFF1:old%=256*X%?4+old%DIV256
   A%=6:X%?4=io%:CALL&FFF1:io%=io%DIV256
   ?X%=?X%+1
 UNTIL ?X%=2
 B%=FNbyte(136,B%,C%)
 !X%=&200:REPEAT
   X%?4=old%:CALL&FFF1:old%=old%DIV256
   ?X%=?X%+1
 UNTIL ?X%=2
 =B%
 DEFFNbyte(A%,X%,Y%)=((USR&FFF4)AND&FF00)DIV256

PROCmem_call(address,"string") will call address, entering with A=0 and XY pointing to string. For example, PROCmem_call(&FFF7,"HELP") calls OSCLI in the I/O processor.

result=FNmem_call(address,param1,param2) will call address, entering with A=1, X=param1 and Y=param2, and will return the result in XY. For example, result=FNmem_call(&FFE7,0,0) will call OSNEWL in the I/O processor.

You should be careful if the called code generate an error as then USERV will not be restored to its previous value.

Implementations

All 8-bit Acorn systems read from whichever ROM is paged in. All except the BBC B+ read from main memory. The B+ recognises &FFFExxxx to read from the shadow screen memory, otherwise it reads from main memory. It is possible to write a modified Tube Host that intercepts OSWORD 5 and 6 to access extended memory, but most programs rely on adding an extra OSWORD call.

See Also

• OSWORD &06
• OSWORD &FA
• OSWORD &FF
• http://mdfs.net/Docs/Comp/BBC/Oswords
• MemIO library at http://mdfs.net/blib

Jgharston 23:56, 30 October 2011 (UTC) Jgharston (talk) 04:27, 29 March 2015 (UTC) Jgharston (talk) 14:01, 31 January 2016 (UTC)