Description: Apple II Hardware: Direct Memory Access (2/97)
Header: Apple II Hardware: Direct Memory Access (2/97)

 Article Created: 21 September 1984
 Article Reviewed/Updated: 28 February 1997

 TOPIC -----------------------------------------------------------

 This article describes DMA (Direct Memory Access) and how it can be used to
 control the Apple's memory and I/O.

 DISCUSSION ------------------------------------------------------

 DMA (Direct Memory Access), is designed to allow the hardware on an interface
 card to completely take over the Apple's memory and I/O. The DMA pin on the
 Apple II interface bus disconnects the Apple's 6502 from the Apple's address
 bus, data bus, R/-W signal, and clock.  The DMA pin should be pulled low
 immediately after phase 0 goes low and should be released when phase 0 again
 goes low. The 6502 is a dynamic device that will lose its registers' memory
 if DMA is held low for more that 40 microseconds. Here is an example where
 the DMA lasts for two microseconds.

 Clock 0 _________            ________            ________
                  |          |        |          |        |
                  |__________|        |__________|        |_____

 DMA     _________                                         _____
                 |                                       |
                 |_______________________________________|

 The 6502 normally sets up its addresses, control, and data while clock 0 is
 low. The memory or I/O device decodes and readies the data while clock 0 is
 high. The actual transfer occurs when clock 0 goes from high back to low.
 During cycles when DMA is low, the user must supply the address, control, and
 data information for that memory cycle in synchronization with the Apple's
 clocks.

 Enabling Unlimited DMA
 ----------------------

 * Point the reset vector to a restart routine, assuming the Apple has the
 Auto-Start ROM.

 * Have the peripheral card generate a RESET when the 6502 is to have control
 of the bus. The Apple II Reference Manual explains the RESET vector starting
 on page 36.


 Article Change History:
 28 Feb 1997 - Reviewed for technical accuracy, revised formatting.
 Copyright 1984-97, Apple Computer, Inc.
 Keywords: