🚀 go-pugleaf

kovarski@zooid.guild.org was heard in "The Pit" saying to  All

  MK> From: Mark Kovarski <kovarski@zooid.guild.org>
  MK> Organization: ZOOiD BBS
  MK> 
  MK> I have the following it of code which was given to me.
  MK> Aside from the "few" errors, the person claims that
  MK> this will cause a message:
  MK> 
  MK> ON BOARD PARITY ERROR
  MK> ADDR (HEX) = (xxxx:xxxx)
  MK> SYSTEM HALTED.
  MK> 
  MK> This apparently appears when the following is run:
  MK> 
  MK> mov al,74
  MK> out 43,al
  MK> mov al,ff
  MK> out 41,al
  MK> mov al,ff
  MK> out 41,al
  MK> ret
  MK> ----------
  MK> I can not check this since I am running OS/2 and traps
  MK> the error before it appears on the screen. The person
  MK> claims that he interfers with the DMA refresh rate and
  MK> this is the reason a parity error appears.
  MK> 
  MK> Could someone check if this is the case and if so, why
  MK> the error appears. From my understanding, a parity error
  MK> can only be caused by DRAM and I/O parity errors. Furthermore,
  MK> on the PC AT, 2BH and 29H are the post addresses for the
  MK> Interrupt controller (8259A). THe Programmer's PC Sourcebook
  MK> mentiones that ports 20 and 21 are only used. There seems to
  MK> be something very strange going on with this program. Thanks...

The values given (74, 43 and 41) are in hex; just like the 'ff'. The first two
commands select Counter 1 (which controls refressh) for a 16-bit read or write
into the rate generator. The next four output FFFFh to the rate generator.
This
effectively stops (actually slows dramatically) memory refresh. Your DRAMs
then
slowly lose their charge and eventually a parity error results.

I would suggest keeping this person away from your PC.   I-{>


 * Origin: Howls from "The Pit" BBS (03) 321-3295 (3:632/456.1@fidonet)