What is DMAMask ?

DMAMask is used for two purposes :-

  1. High speed devices like disk controllers can read data straight from memory without any CPU load. This access is call direct memory access or DMA.

    Standard Amiga devices can only use CHIP memory for DMA. Disk controllers are usually capable of using DMA to any memory.

    The DMAMask value restricts DMA activity by a particular device driver to a particular area of memory.

    The default value of 0xFFFFFF restricts DMA to the Zorro-II area. This is a bad idea for any Zorro-III based machine which always has memory outside this area. In this case 0xFFFFFFFF would be a better value.

  2. The DMAMask also control the alignment of any buffers used to access the device. The least significant bits are used to control this. Values with no zero bits, end in 0xF, will have no alignment. Values with one zero bit, end in 0xE, will be word aligned. Values with two zero bits, end in 0xC, will be long word aligned.

    The alignment can make a lot of difference to performance. Longword alignment is usually the best.

Follow this link for instructions on selecting the DMA Mask value.