Date   : Mon, 15 Aug 1994 13:33:29 EST
From   : Stephen Quan <quan@...>
Subject: Re: i see basic! :-)

>> 386DX-33 it is operating at about 30 times slower than a real beeb.

> 30 times slower?!? That's pretty awful (no offence)! I would have thought 
> that even a C version would be a bit faster than that. Although I have 
> yet to get BASIC going, I would have thought that it would go no slower 
> than about 1/2 Beeb speed. However, I might retract that when I get it going!

I tried a 256^2 loop (LDX #0 LDY #0 DEY BNE DEX BNE), and it took about
30 seconds to run.  Oh geeze, that means I am 1/256 times slower, not 1/30.
Hmmrph, mumble, grumble.  ( :-) )

I have pretty awful memory read/write routines as it was not possible to
create one flat 64K block of memory using simple Borland C.  I opted to
have 32K for RAM, 16K for the OS and 16*16K for the ROMs.  I had to
navigate through ifs statements to determine which array I should be
accessing, and (in the case of ROM), whether I should write to there.

In my assembly version I can create a 64K flat block of memory with
no hassles.  As a matter of fact, I decided to model it using the EMS
page frame (as I proposed to do so earlier!).  I have

    DS = EMS page frame
    SI = 6502 PC

And load opcodes and operands using LODSB and LODSW.  My memory map is :

    EMS logical page 0 = user memory 0000-3FFF (EMS page 16).
    EMS logical page 1 = user memory 4000-7FFF (EMS page 17).
    EMS logical page 2 = sideways RAM/ROM      (EMS page 0-15).
    EMS logical page 3 = OS                    (EMS page 18).

But, because of this and of other things I have done in my code, I
cannot run my emulator on anything less than a 386.

ALSO, in the name of performance, I have opted to restrict ROM special
behaviour only for LDA/LDX/LDY and STA/STX/STY instructions, meaning
that other instructions will see the 64K as a flat piece of memory.
This speeds up performance by not having to always check for the ROM
special behaviour for nearly every memory related command.  There is
also a risk of somebody (using the emulator) changing the ROMs during
run time using the ROL command.  If this looks bad, I may have to
extend ROM special behaviour to include other commands as well.

I hardly see it necessary to implement ROM special behaviour for
straight 6502 fetch and execute cycle.
-- 
Stephen Quan (quan@...                 ), SysAdmin, Analyst/Programmer.
Centre for Spatial Information Studies, University of Tasmania, Hobart.
GPO BOX 252C, Australia, 7001.  Local Tel: (002) 202898 Fax: (002) 240282
International Callers use +6102 instead of (002).