ISA DMA: Difference between revisions

 

 

In the beginning there was a PC, but the PC was slow. IBM looked down from the heavens and said "Slap on a DMA controller - that should speed it up." IBM's heart was in the right place, its collective brains were elsewhere as the DMA controller never met the needs of the system. The PC/AT standard contains 2 Intel 8237A DMA chips, connected as Master/Slave. The second chip is Master, and its first line (Channel 4) is used by the first chip, which is Slave. (This is unlike the interrupt controller, where the first chip is Master.) The 8237A was designed for the old 8080 8-bit processor and this is probably the main reason for so many DMA problems. 8088 and 8086 processors chosen by IBM for its PC were too advanced for the DMA controller.