On Fri, 30 Apr 1999 04:03:52 -0230, James Oakley wrote:
"Timothy J. Massey" wrote:
Isn't the ISA bus usually limited to 8MHz? I've seen ISA busses overclocked as high as 16MHz, but never any higher. So, a 20MHz scope should show you how fast the CPU is accessing the BIOS.
You're right. I was wondering how fast the BIOS was accessed, ISA speed or something closer to memory speed (I think it's ISA speed, but I'll check anyway). If it's ISA speed, the board will be much simpler as we can just wire the signals from the ISA bus.
After I reread your message, I figured that was what you were asking. But here's my question, I guess: what do you mean, "We can just wire the signals from the ISA bus." How can you just pick what signal wires you use? If the BIOS needs line x, you need to give it x! And if the BIOS signal wires are already using ISA bus signal lines, then why would you have had a question about how fast the CPU access the BIOS? In that case, the access speed to the BIOS would *have* to be ISA bus speeds!
Of course, my understanding is not relevant to the working of your circuit. If you don't feel like wasting your time on my understanding, I would understand. ;)
Tim Massey
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After I reread your message, I figured that was what you were asking. But here's my question, I guess: what do you mean, "We can just wire the signals from the ISA bus." How can you just pick what signal wires you use? If the BIOS needs line x, you need to give it x! And if the BIOS signal wires are already using ISA bus signal lines, then why would you have had a question about how fast the CPU access the BIOS? In that case, the access speed to the BIOS would *have* to be ISA bus speeds!
Of course, my understanding is not relevant to the working of your circuit. If you don't feel like wasting your time on my understanding, I would understand. ;)
You can take a small ISA-prototype-card and a address-decoder (e.g. 74HC245). With these 2 things you can connect a flash/eprom/prom/whatever to a specific location in the system-memory (first 16meg, i think on isa-bus)
ciao, stephan
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On Fri, 30 Apr 1999, Stephan Müller wrote:
You can take a small ISA-prototype-card and a address-decoder (e.g. 74HC245).
Sorry, the 74HC245 is no address decoder, it's an 8 bit parallel buffer driver (usually used for the data bus).
With these 2 things you can connect a flash/eprom/prom/whatever to a specific location in the system-memory (first 16meg, i think on isa-bus)
You will need a real address decoder for that. If size is the problem, use a GAL.
Sehr Wus, - Matthias
-----Ursprüngliche Nachricht----- Von: owner-openbios@elvis.informatik.uni-freiburg.de [mailto:owner-openbios@elvis.informatik.uni-freiburg.de]Im Auftrag von Matthias Wächter Gesendet am: Freitag, 30. April 1999 15:12 An: openbios@elvis.informatik.uni-freiburg.de Betreff: Re: AW: [OpenBIOS] OT: Bios access speeds
On Fri, 30 Apr 1999, Stephan Müller wrote:
You can take a small ISA-prototype-card and a address-decoder (e.g. 74HC245).
Sorry, the 74HC245 is no address decoder, it's an 8 bit parallel buffer driver (usually used for the data bus).
Hm, what else is used? I can only see this 74245 everywhere... :)
With these 2 things you can connect a flash/eprom/prom/whatever to a specific location in the system-memory (first 16meg, i think on isa-bus)
You will need a real address decoder for that. If size is the problem, use a GAL.
or so. :)
CyA, Stephan
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Stephan M�ller wrote:
You can take a small ISA-prototype-card and a address-decoder (e.g. 74HC245).
Sorry, the 74HC245 is no address decoder, it's an 8 bit parallel buffer driver (usually used for the data bus).
Hm, what else is used? I can only see this 74245 everywhere... :)
Address decoders are 74LS138 (3 to 8) and 74LS139 (dual 2 to 4). I can remember almost all the chips in 74 series. Newer motherboards usually don't use them (build the functions in LSI/VLSI/FPGA).
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On Fri, 30 Apr 1999, Qiwei Xiao wrote:
Sorry, the 74HC245 is no address decoder, it's an 8 bit parallel buffer driver (usually used for the data bus).
Hm, what else is used? I can only see this 74245 everywhere... :)
Address decoders are 74LS138 (3 to 8) and 74LS139 (dual 2 to 4). I can remember almost all the chips in 74 series. Newer motherboards usually don't use them (build the functions in LSI/VLSI/FPGA).
The Problem with the '138 is, that it just decodes 3 address lines where one needs much more, but outputs 8 where one typically needs 1 or 2. So I think, a GAL is better suited, additionally because it's reprogrammable.
Qiwei
Sehr Wus, - Matthias
On Fri, 30 Apr 1999, Stephan Müller wrote:
You can take a small ISA-prototype-card and a address-decoder (e.g. 74HC245).
Sorry, the 74HC245 is no address decoder, it's an 8 bit parallel buffer driver (usually used for the data bus).
Hm, what else is used? I can only see this 74245 everywhere... :)
Well ... I can't answer that in general. You will see the 74(xx)245 on various ISA-Boards, because it is needed to be there. The ISA Bus is not capable of driving that much input pins (of every ISA card) in parallel, so transceivers are (to be) used which are only switched on (and thus loading the bus) if the device is selected. Additionally it is used to multiplex address and data bus for some chips multiplexing these signals.
Anyway, the '245 is nothing more than an enablable two-way amplifier.
You will need a real address decoder for that. If size is the problem, use a GAL.
or so. :)
Well, my TTL times are gone for long, so I can't tell you a type number from my mind. But anyway, GALs are really cheap nowadays and one can place it on a socket to reuse it in any other, different place and application.
Sehr Wus, - Matthias
On Fri, 30 Apr 1999, Matthias Wächter wrote:
On Fri, 30 Apr 1999, Stephan Müller wrote:
You can take a small ISA-prototype-card and a address-decoder (e.g. 74HC245).
Sorry, the 74HC245 is no address decoder, it's an 8 bit parallel buffer driver (usually used for the data bus).
With these 2 things you can connect a flash/eprom/prom/whatever to a specific location in the system-memory (first 16meg, i think on isa-bus)
You will need a real address decoder for that. If size is the problem, use a GAL.
And you should make sure the device is accessible after reset. I don't know much about the relevant chipsets, but you should definitely make sure that the processor's reset vector is in the bios address range.
And you should probably make sure that the ISA bus works after reset. I never did something with real motherboards, but I hacked something to load Linux on an embedded system using amd's elan chip (486 core with integrated peripherals). And the elan has only a single chip select configured after reset, to use ISA, the bios has to activate it before.
Peter
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Matthias Wächter -
Peter Pointner -
Qiwei Xiao -
Stephan Müller -
Timothy J. Massey