AGP question

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pigdos

I learned in my basic digital logic and design courses that the clock rate
determined the rate at which flip-flops/registers could read in data. My
question is how can a GPU running at say, 500Mhz, clock-in AGP data at a
rate of 2+GB/sec? I suppose if the registers clock in data on the rising and
falling edges it wouldn't be an issue. Is this what happens?
 
I learned in my basic digital logic and design courses that the clock rate
determined the rate at which flip-flops/registers could read in data. My
question is how can a GPU running at say, 500Mhz, clock-in AGP data at a
rate of 2+GB/sec? I suppose if the registers clock in data on the rising and
falling edges it wouldn't be an issue. Is this what happens?
Click to expand...

You're assuming that the bus width is 8 bits/ 1 byte.
 
I learned in my basic digital logic and design courses that the clock rate
determined the rate at which flip-flops/registers could read in data. My
question is how can a GPU running at say, 500Mhz, clock-in AGP data at a
rate of 2+GB/sec? I suppose if the registers clock in data on the rising and
falling edges it wouldn't be an issue. Is this what happens?
Click to expand...

Hmm AGP bus is 32bit. So 500Mhz x 32 bit at single data rate is just
nice 2GB/sec. :ppPp
 
The little lost angel said:
Hmm AGP bus is 32bit. So 500Mhz x 32 bit at single data
rate is just nice 2GB/sec. :ppPp
Click to expand...

True enough, but actually there's more headroom: IIRC, plain
AGP is 64bit * 66 MHz = 528 MB/s [peak, outside of longish setup]
AGP does go to 8x = 4.2 GB/s bus limit. If GPU is 64+bit, then
500 MHz GPU limit is 4.0 GB/s, assuming one clock per xfr.

The GPU's bus is mostly busy hammering on vid.ram. DMA [busmaster]
transfer to system RAM are usually infrequent, which is one reason
why plain PCI vid.cards are sold (The other reason is for expansion
of PCIe- & AGP-less econoboxes). When transfer between system &
vid.RAM _is_ important (software MPEG2 decoding or other attempts
to use the CPU for GPU work), then systems often cannot keep up.


-- Robert
 
The little lost angel said:
Hmm AGP bus is 32bit. So 500Mhz x 32 bit at single data
rate is just nice 2GB/sec. :ppPp
Click to expand...

True enough, but actually there's more headroom: IIRC, plain
AGP is 64bit * 66 MHz = 528 MB/s [peak, outside of longish setup]
AGP does go to 8x = 4.2 GB/s bus limit. If GPU is 64+bit, then
500 MHz GPU limit is 4.0 GB/s, assuming one clock per xfr.
Click to expand...

I think you're thinking of PCI-X. AGP is 32-bits address & data + 8-bits
SBA. For AGP 3.x 8x, the common clock is 66MHz, the source synchronous
strobe clocks are 4x that with DDR giving 533MT/s for a peak bandwidth for
AGP 8x of 2.1GB/s.
 
I see. So obviously any modern GPU can more than keep up w/AGP. Is this one
reason PCI express is the superior tech?
 
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