I have a few questions regarding the DMA lab:

Does the CPU write directly in the registers (eg, the control register bits such as R/W, burst mode or the Data Register (for a write).

When does the DMA Controller know when to start operating and request the bus? Is this when the count value is > 0?

When a write is being performed, does the CPU insert the starting address in the address register?

Thanks,

-- Shezad Okhai (shezad.okhai@utoronto.ca), February 16, 1999

Answers

The DMA Controller should start operating when the count is > 0. This was meant to be mentioned in the handout, but somehow got left off. Sorry.

The basic flow using the DMA Controller is as follows:

• CPU sets base address by writing to ADDR_REG
• If write is to be performed, set value to be written by DMA Controller by writing to DATA_REG
• Set the type of operation and number of transfers by writing appropriate values to CONT_REG. (Note that a non-zero value for count should start DMA process.
• After transfer is complete, you can check either the relevent memory locations (for write) or DATA_REG (for read) to see the transfer was successful.

-- Steven Caranci (caranci@eecg.toronto.edu), February 18, 1999.