Date   : Sun, 05 Jan 1992 06:06:16 GMT
From   : taco!rock!lester.appstate.edu!stat.appstate.edu!pembvax1.pembroke.edu!duck@lll-winken.llnl.gov
Subject: Re: C128 BDOS question -- Faster CP/M???

In article <1992Jan04.185759.8045@microsoft.com>, kentsu@microsoft.com
(Kent SULLIVAN) writes:
> In article <1992Jan4.080346.4320@pembvax1.pembroke.edu> duck@pembvax1.pembroke.edu
writes:
>>
>>  I was recently llooking through the source code of the BDOS for the
C128 CP/M
>>and noticed that they had an option for use$fast -- apparently to speed
up the
>>Z80 into 4 mhz mode from 2 mhz by toggling the clock speed at the appropriate
>>points (ie: the clock speed can't be 4mhz when switching processors and a
>>couple of other similair situations).  However, IT DOESN'T WORK.  I changed
the
>>equate to true (file cxequ.lib). And recompiled the whole thing (talk about
>>growing old) and no go.
> 
> [Discussion of use$fast turnning off the 40col screen on bootup]
> 
> I think that possibly $usefast = true also puts the 8502 into 2 Mhz. mode, so
> that the times when it is turned on things run faster.  Of course, much
of the
> time when it is on, I/O is being done (like to the disk), which means things
> are stepped down to 1 Mhz. again.
> 
> [Discussion of way to check if 40col screen is being turned off on bootup]

  Nope, The 40col screen is not affected by booting CP/M after compiling it
with use$fast.  The code that I've looked at (briefly - I like 6502 better)
looked like it was saving the system speed (at 53296 regular 128 mode -- 4get
the hex address) and toggling to 1mhz mode and then doing the opposite after
some section of code.  However, apparently the system speed is never set to 2/4
mhz and so useless swapping is done.  I tried adding in a set routine but it
bombed right before it gave me the command prompt.

> 
> I believe the Z80 is actually running at 4 Mhz. but that the architecture of
> the computer is such that it only runs for a fraction of each second.  The
> rest of the time is given to other chips in the computer.   So the _effective_
> rate is 2 Mhz.

  Okay, (After checking the C128 programmer's reference guide). The C128 gets
its 4mhz clock from the VIC chip during AEC low (in other words, it's working
1/2 the time) .... Now, my question is why is use$fast in there???
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

               {Note crosspost to comp.os.cbm}

> -- 
> Kent Sullivan - Microsoft Corporation  Redmond, WA - kentsu@microsoft.com
> 
> The opinions expressed in this article are my own and do not necessarily
> reflect those of my employer.

- Craig Taylor
duck@pembvax1.pembroke.edu