Etienne Lorrain (etienne_lorrain.delete@this.yahoo.fr) on March 5, 2021 6:23 am wrote:
> Carson (carson.delete@this.example.edu) on March 5, 2021 2:31 am wrote:
> > Etienne Lorrain (etienne_lorrain.delete@this.yahoo.fr) on March 4, 2021 6:58 am wrote:
> > > You cannot "only initialising a page when it was allocated" because the BIOS do not allocate
> > > memory, it just tells you how much memory is available, and the concept of page is not known
> > > (no virtual memory at that point). Moreover the page size is decided by the OS, processor support
> > > different sizes. Moreover memory over 1 Megabyte was special, either EMS or XMS or HMA.
> > > And you could not modify DOS, proprietary software with no source.
> > > The real missing thing was (and still partly is) a proper DMA, able to send more than 64 Kbytes
> > > and able to access more than 1 Megabyte / 16 Mbytes with chipset recognition and NDA docs.
> >
> > These all seem like non-issues.
> >
> > For OSes which cannot handle uninitialized ECC, have a forward-compatibility flag somewhere
> > in the boot loader which means "I can handle uninitialized ECC". If it's not present, BIOS
> > clears memory before transferring to the boot loader. A chaining boot loader (LILO or whatever)
> > is required to do the same check on OSes is chain-loads. (By calling back into the BIOS which
> > has all the necessary code, so the chain loader doesn't suffer much code bloat.)
> >
> > For loaders which do support uninitialized ECC, a simple BIOS data structure (like the 0xE820 memory
> > map) describes the initialized parts, and there are BIOS calls to extend the initialized parts.
> >
>
> You make me feel old, that is bad on Friday -:)
> BIOS was developed at a time where Linux was a sin, you would be made immediately redundant
> if you installed it on a PC at work (because there was no anti-virus on Linux).
> Because DOS did not manage ECC at all, and Windows 3.1/95 started from
> DOS, the BIOS had to initialise all the memory it knew about.
> DOS did not know about EMS or XMS memory, nothing more than 1 Mbyte.
> EMS could be provided by an ISA card (i.e. something like PCI), and video/network card
> BIOS would be on the ISA/PCI card itself, their BIOS did not manage ECC either.


A point - essentially none of the early PCs had ECC, although many had parity checking on main memory. ECC is expensive if the memory is less than about 64 bits wide (you need seven bits on 32-bit wide RAM, six bits on 16-bit wide ram, five bits on byte-wide RAM). The PC/XT had 8-bit parity protected RAM, and the AT did 16-bit RAM, with parity on each byte. If memory serves, parity errors triggered an NMI on both the PC and AT class systems.

Parity as a standard desktop PC feature was dropped before it ever really turned into ECC. Many 386 systems did not have it. Some 286 system did not either. The original Macs didn't have parity either.