Thread: Amiga Vs ST
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Old 30 July 2009, 03:22   #181
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Originally Posted by Ricardo View Post
The Amiga's hardware also stole cycles off the 68K, particularly with the blitter running full blast, but there was still the opportunity for some parallel processing although I doubt the hardware in the Amiga would have been much use for 3D modelling hence David Braben's comments re: ST CPU speed.

The 68000 uses only the even-numbered memory access cycles. The 68000 spends about half of a complete processor instruction time doing internal operations and the other half accessing memory. Therefore, the allocation of alternate memory cycles to the 68000 makes it appear to the 68000 that it has the memory all of the time, and it will run at full speed.

However some 68000 instructions do not match perfectly with the allocation of even cycles and cause cycles to be missed. If cycles are missed, the 68000 must wait until its next available memory slot before continuing. However, most instructions do not cause cycles to be missed, so the 68000 runs at full speed most of the time if there is no blitter DMA interference.

If the display contains four or fewer low-resolution bit-planes, the 68000 can be granted alternate memory cycles (if it is ready to ask for the cycle and is the highest priority item at the time). However, if there are more than four bit-planes, bit-plane DMA will begin to steal cycles from the 68000 during the display.

Some of that was a quote from the Amiga Hardware Reference manual .
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