::: Area #157 (comp.sys.acorn.programmer)
Message: #28252 (Read 10 times, has 0 replies, 1364 bytes)
Date   : Sat Jan 31 11:23:53 1998
From   : Jonathan Graham Harston of fidonet#2:254/27
To     : All
Subject: Subject: SYS calls/CALLs

MG> as CALL &FFF1, is very machine architecture specific - it requires the
MG> OSWORD routine to be at &FFF1, which is impossible on an ARM

All versions of BBC BASIC have in their CALL/USR routine a bit of code that
does the following:

IF (address AND &FFFFFF00)<>&FF00 OR
   CALL has parameters THEN
     Set registers from A%,B%,C%,D%,E%....
     CALL the address
     Return result from primary register
   ELSE
     Set registers from A%, X%, Y%, C%
     Perform the required MOS interface call
     Return the result from Cy:X:Y:A
ENDIF

...where of course in 6502 BBC BASIC the ELSE clause is identical to the
THEN clause, and so factorises out.  Quoting the manual (from memory)...

Addresses &0FFxx are used to access Operating System calls. However, these
addresses are entirely possible as true assembly language entry points and
must therefore be avoided when using the assembler. This problem may be
circumvented by using CALL with at least one parameter (in addition to the
address) which will enter an assembly language routine even if its address
is in the range &FF00 to &FFFF.

J.G.Harston (BBC PD Library) jgh@arcade.demon.co.uk
70 Camm Street, Walkley, Sheffield, S6 3TR
--- ARCbbs RISC OS [1.64á32c]
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