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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: freqmax on January 16, 2013, 11:14:54 AM
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What is the bus frequency limit for Amiga without CPU ?
Ie what is the limit without any CPU to throttle.
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Which bus? The main system clock is 28.37516Mhz, the CPU and other things just divide that down for their needs.
ZIII is asynchronous, so I guess it will work at whatever clock the device driving it at the time wants to run it at.
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I were mainly thinking on the local bus that is physically wired to Agnus (OCS/ECS/AGA), Paula and Denise etc.
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I were mainly thinking on the local bus that is physically wired to Agnus (OCS/ECS/AGA), Paula and Denise etc.
Probably linked to the pixel clock... The fastest I remember is something like 71ns... So about 14Mhz I guess :-?
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What is the bus frequency limit for Amiga without CPU ?
Ie what is the limit without any CPU to throttle.
I were mainly thinking on the local bus that is physically wired to Agnus (OCS/ECS/AGA), Paula and Denise etc.
For access to chip ram it's 7mhz for OCS/ECS & 14mhz for AGA. The CPU only gets spare cycles, so the limit is irrelevant to whether there is a CPU or not.
The CPU only has access to Agnus, which has it's own register bus that connects to paula and denise. I believe the register bus runs at the same speed as chip ram.
1. dma'd data from chip ram gets sent across this bus, so it has to be as fast as chip ram.
2. There would be no benefit to have run it at a faster clock, because nothing could have made use of it.
However it also suffers contention from dma & copper in Agnus. I don't know whether the contention of the register and chip ram bus is handled separately or whether it's combined.
Is that what you wanted to know?
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The Zorro bus on an Amiga 2000 is 7mhz at 16-bit.
Not sure about Zorro III, but it varies.
The chipset is mostly sound and graphics and can't really be sped up. If you add fast RAM/CPU RAM this takes a load off the chip RAM and gives you a speed up.
Are you asking about bottlenecks? E.g. You have a fast graphics card, but after a certain point it starts to slow down. This would because the Zorro bus can't feed enough data from the CPU to the graphics card.
Sometimes a faster CPU might give you a speed increase. It depends on whether a task is graphics intensive or CPU intensive.
If you want faster you have to get as much of your expansions close to the CPU. The IDE interface on an A1200 is particular slow, put it next to a fast CPU and it would be doing 10 Megabytes a second or more.
It's old hardware so I guess there is no point in trying to squeeze more out of the chipset.
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For access to chip ram it's 7mhz for OCS/ECS & 14mhz for AGA. The CPU only gets spare cycles, so the limit is irrelevant to whether there is a CPU or not.
The CPU only has access to Agnus, which has it's own register bus that connects to paula and denise. I believe the register bus runs at the same speed as chip ram.
I think I read somewhere that it can access Agnus at 28Mhz.
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I think I read somewhere that it can access Agnus at 28Mhz.
I would like to see where you read that.
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Actually it works at 28mhz internally as a co-processor. That sounds right.
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So clocking the CPU socket (bus) faster than 28 MHz is pointless except for any onboard memory (60 ns dram?).
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So clocking the CPU socket (bus) faster than 28 MHz is pointless except for any onboard memory (60 ns dram?).
With higher clock speeds RAM becomes a huge (tiny) bottleneck. That's why today's systems have large caches and synchronous memory with ever increasing clocks.
My Phenom uses more than 8 MB cache - a completely ridiculous amount if you think about it and nearly half the total amount of memory in my A3k.
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So clocking the CPU socket (bus) faster than 28 MHz is pointless except for any onboard memory (60 ns dram?).
That depends, A 68000 doesn't access ram on every cycle.
With the cheap 14mhz 68000 hack, it has to synchronise with the 7mhz clock when accessing the bus. So quick instructions aren't really affected, but long instructions like MULT/DIV can run quicker. But you don't get faster chip ram or access to registers.
For an 020 the code could be running from cache and not accessing any ram at all. But generally if you have an accelerator then you need local fast ram too.
For big box Amiga's, Zorro 2 cards have to be accessed at 7mhz but Zorro 3 cards have no fixed clock in the specification so they should run quicker.
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For big box Amiga's, Zorro 2 cards have to be accessed at 7mhz but Zorro 3 cards have no fixed clock in the specification so they should run quicker.
That's true, but big box Amiga CPU's have a 25MHz (16Mhz on some 3k's) connection to the motherboard, so that's one limit.
The other is the fact that Buster and (Bridgette if preset) can't run fast enough to fully use the already available bandwidth.
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Looking at the A1200 (http://www.amiga-hardware.com/download_photos/a1200mb_rev1d4.jpg) motherboard I think the datasheet for the "CSG 8374" would probably answer my questions ;)
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That's true, but big box Amiga CPU's have a 25MHz (16Mhz on some 3k's) connection to the motherboard, so that's one limit.
A2000 don't, but yeah A3000 and A4000 have a 50mhz clock on the motherboard. I'd assumed it was used for something else, but it could limit access to 25mhz. There is a clock source jumper, but I don't know what it's for.
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A2000 don't, but yeah A3000 and A4000 have a 50mhz clock on the motherboard. I'd assumed it was used for something else, but it could limit access to 25mhz. There is a clock source jumper, but I don't know what it's for.
Sorry, I always forget 2000's as a "big box" since they are essentially glorified A500's. ;)
I'm 99% sure it's a 25MHz on the motherboard. Lots of 040s have a 50MHz because that CPU double clocks, but I'm not aware of one on any motherboards.
The clock jumpers determine whether the it uses the clocks from the motherboard or the ones on the CPU card.
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The clock jumpers determine whether the it uses the clocks from the motherboard or the ones on the CPU card.
Yeah, so with a 33mhz CPU card is there still a 25mhz limit?
The crystal on the motherboard is either a 32mhz for a 16mhz 68030 or 50mhz for a 25mhz 68030.
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The 28 MHz clock is divided in to 7 MHz for OCS/ECS and 14/7 MHz for AGA. There is nothing actually on the chip bus running from a 28 MHz clock. The chip bus can be overclocked to some extent but it will mess up the Video signal timing.
The 7 MHz Zorro2 and 25 MHz Zorro3 buses could also be overclocked to some extent, but some Zorro2 cards will fail and Zorro3 cards won't realize much performance improvement since they operate from their own (asynchronous) clocks.