Program the DMA Controller for Memory Transfers

ISA DMA controller

The 8237 DMA chip moves bytes between I/O devices and memory without CPU copying. Program channel registers for address, count, and mode, then assert mask/unmask bits. For example, floppy controllers historically used DMA channel 2 to transfer 512-byte sectors to 0x7C00.

Channel setup outline

nasmCopy
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• Ports: 0x00-0x0F master, 0xC0-0xDE page registers for high address byte
• Mask register: Disable channel during programming
• Count register: Actual value is count minus one
• ISA limitations: 24-bit addresses unless 16-bit DMA page registers used

Floppy and alignment

Historical DMA channel 2 for the floppy required word-aligned buffers in low memory. The page register holds bits 16-23 of the address on ISA machines. Unmask only after all bytes of the transfer count are written; the flip-flop resets between low and high byte writes to the count port. QEMU still models ISA DMA enough to see a transfer complete bit assert in the master status register.

Why for this exercise

You will program a DMA channel for a memory buffer and trigger a device transfer. This exercise requires writing address/count bytes in the correct order and unmasking the channel when ready.