AMD Am186 ES, User Manual, Page: 141 / 192

DMA Controller 9-9
9.3.6 DMA Source Address High Register (High Order Bits)
(D0SRCH, Offset C2h, D1SRCH, Offset D2h)
Each DMA channel maintains a 20-bit destination and a 20-bit source register. Each
20-bit address takes up two full 16-bit registers (the high register and the low register) in
the peripheral control block. For each DMA channel to be used, all four address registers
for that channel must be initialized. These addresses can be individually incremented or
decremented after each transfer. If word transfers are performed, the address is
incremented or decremented by 2 after each transfer. If byte transfers are performed, the
address is incremented or decremented by 1.
Each register can point into either memory or I/O space. The user must program the upper
four bits to 0000b in order to address the normal 64K I/O space. Since the DMA channels
can perform transfers to or from odd addresses, there is no restriction on values for the
destination and source address registers. Higher transfer rates can be achieved on the
Am186ES microcontroller if all word transfers are performed to or from even addresses so
that accesses occur in single 16-bit bus cycles.
Figure 9-6 DMA Source Address High Register
The value of D0SRCH and D1SRCH at reset is undefined.
Bits 15–4: Reserved
Bits 3–0: DMA Source Address High (DSA19–DSA16)—These bits are driven onto A19–
A16 during the read phase of a DMA transfer.
15
7 0
Reserved
DSA19–DSA16