On 10/22/2010 07:25 PM, Rich Hare wrote:
Steven Hirsch wrote:On 10/21/2010 12:50 PM, Rich Hare wrote:I've read my RamFast manuals and can't seem to find anything about "polled transfer mode". There are no dip switches on the RamFast D that I can find.I could have sworn there was a setting to turn off DMA transfers. If the card has no dipswitches, it's probably accessed through the software somehow.Thank you Steven... I still don't know about "polled transfer mode", but, YES, there is a selection on the built-in software to turn DMA off.
When you turn DMA off, the unit works in polled mode. DMA is "Direct Memory Access". In that mode, the card takes over the system bus, halts the processor and clocks data directly into the motherboard memory. When the processor wakes up, an entire block is "magically" then available. In polled mode, the processor transfers a block of data itself by spinning in loop and reading/writing a byte at a time from/to a register on the card.
DMA is generally faster since every memory access cycle moves one byte. When you poll, the CPU has the additional overhead of fetching and decoding the instructions in the loop. A single instruction can take multiple cycles and not every instruction actually moves data. On top of that, the code typically checks a status bit in another register to see if data is ready and may burn cycles waiting if it is not.
ALAS, I turned DMA off with no resultant difference.
Well, it was worth trying! Hope you get it soon. Steve