Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

David P. Reed
2007-12-11T23:57:19Z

1) I found in a book, the Undocumented PC, that I have lying around that
the "pause" recommended for some old adapter chips on the ISA bus was 1
usec. The book carefully points out on various models of PCs how many
short jumps are required to implement 1 usec, and suggests that for
faster machines, 1 usec loops be calibrated. That seems like a good
heuristic.

2) Also, Dick, you got me interested in doing more historical research
into electrical specs and circuit diagrams (which did come with the IBM
5150). The bus in the original IBM PC had no problem with "bus capacity
being charged" as you put it. Perhaps you don't remember that the I/O
bus had the same electrical characteristics as the memory bus. Thus
there is no issue with the bus being "charged". The issue of delays
between i/o instructions was entirely a problem of whether the adapter
card could clock data into its buffers and handle the clocked in data in
time for another bus cycle. This had nothing to do with "charging" -
buses in those days happily handled edges that were much faster than 1 usec.

We at Software Arts did what we did based on direct measurements and
testing. We found that the early BIOS listings were usually fine, but
in fact were misleading. After all, the guys who built the machine and
wrote the BIOS were in a hurry. There were errata.

linux-os (Dick Johnson) wrote:
> You do remember that the X86 can do back-to-back port
> instructions faster than the ISA bus capacity can be
> charged, don't you? You do remember the admonishment
> about:
> intel asm
>
> mov dx, port ; One of two adjacent ports
> mov al,ffh ; All bits set
> out dx,al ; Output to port, bits start charging bus
> inc al ; Al becomes 0
> inc dx ; Next port
> out dx,al ; Write 0 there, data bits discharged
>
> When the port at 'port' gets its data, it will likely
> be 0, not 0xff, because the intervening instructions
> can execute faster than a heavily-loaded ISA bus.
>
>