The SC1100 controller requires 4-byte aligned data transfers and cannot handle transfers of exactly 64 kilobytes.
The CS5530 multifunction chip/core's IDE section claims to be capable of UDMA mode 2 (33.3MB/s) but in practice using that mode swamps the controller so badly that
geodeide limits the UDMA negotiation to mode 1 (25MB/s) so that the other functions of this chip continue to work.
The IDE DMA engine in the CS5530 can only do transfers on cache-line (16-byte) boundaries. Attempts to perform DMA on any other alignment will crash the system. This problem may also exist in the SC1100 since the CS5530 was its direct predecessor, and it is not clear that National Semiconductor fixed any bugs in it.
The
geodeide driver will reject attempts to DMA to buffers not aligned to the required boundary. The
wd(4) disk driver will back off to PIO mode to accomplish these transfer requests, at reduced system performance.