Date   : Wed, 27 Oct 2004 12:24:33 +0200
From   : John Kortink <kortink@...>
Subject: Re: &D00

On Wed, 27 Oct 2004 10:57:34 +0100, Richard Gellman wrote:

>[...]
>
>Eep! Don't want to RTI. On BBC Micros, there is almost never a need for
>a user program to use RTI. The 6502 guns all NMIs through the NMI vector
>address at the top end of the MOS ROM. This vector points to a location
>in the MOS which in turn indirects through soft NMI vectors.

Eh ? There's no soft vector anywhere in NMI handling.
NMI jumps to (&FFFA) which is &0D00 on both BBC and
Master.

>The MOS calls this vector  through a normal JSR-JMP construct[1], so
>return should be by RTS, not RTI.

Eh again ?

>The difference between the two being of course that RTI also pops the
>status register off the stack in addition to the return address.
>
>The same applies for software interrupts (yes, the 6502 has them.. its
>the BRK instructions) and IRQs. In fact, the 6502 sends IRQ and BRK
>through the same ROM vector, then MOS distriguishes which it is, and
>calls the appropriate software vector (BRKV. IRQ1V, or IRQ2V).  NMIs are
>actually vectored slightly differently due to their nature. The MOS only
>permits "registered" entry points to intercept NMIs, which is why only
>the disk and net controllers use them (and also because they need to
>respond to service requests regardless of CPU state).

Triple eh ? What system are you talking about ? Certainly
not a regular BBC or Master.

>[1] The JSR-JMP construct is a cunning way of implementing JSR (addr).
>The basic code runs as follows:
>
>Store JMP (addr) opcode (&6C):
>LDA #&6C:STA &1000
>Store the address:
>LDA #LSBaddr: STA &1001
>LDA #MSBaddr: STA &1002
>Then JSR it:
>JSR &1000
>
>The JMP jumps to the code, but the RTS will return to the address the
>JSR was called from.

I don't know, cunning ? A well known, better (memory frugal,
shorter, faster, ...) way is :

LDA #MSB(addr - 1):PHA
LDA #LSB(addr - 1):PHA
RTS

and JSR to that. Supposing <addr> is not known until
runtime that is.


John Kortink

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