80C186EC/80C188EC Microprocessor User's Manual

DIRECT MEMORY ACCESS UNIT
XOR AX, AX ; HIGH ADDRESS=0
MOV DX, D2DSTH
OUT DX, AX
; THE POINTER ADDRESSES HAVE BEEN SET UP. NOW WE SET UP THE TRANSFER COUNT.
MOV AX, 25 ; THE MESSAGE IS 25 BYTES LONG.
MOV DX, D2TC ; XFER COUNT REG
OUT DX, AX
; SELECT THE SERIAL PORTS AS THE SOURCE OF INTERNAL DMA REQUESTS
; AND SELECT MODULE B AS THE HIGHEST PRIORITY MODULE.
MOV DX, DMAPRI
MOV AX, 0404H ; IDRQB=1, DMAPB=1
OUT DX, AX
; NOW WE NEED TO SET THE PARAMETERS FOR THE CHANNEL AS FOLLOWS:
;
; DESTINATION SOURCE
; ----------- ------
; I/O SPACE MEMORY SPACE
; CONSTANT PTR INCREMENT PTR
;
; TERMINATE ON TC, INTERRUPT, SOURCE-SYNCHRONIZED, LOW PRIORITY RELATIVE
; TO CHANNEL 1, BYTE XFERS. INTERNAL DRQ. ARM CHANNEL.
MOV AX, 0001011101010110B
MOV DX, D0CON
OUT DX, AX
; THE TRANSMIT CHANNEL IS NOW ARMED. IT WILL NOT BEGIN TRANSFERS UNTIL
; IT IS "PRIMED" BY SENDING THE FIRST BYTE MANUALLY.
; NOW SET UP CHANNEL 4 TO HANDLE RECEIVE REQUESTS FROM SERIAL CHANNEL 1.
MOV
AX, SEG RECV_BUFF
ROL AX, 4 ; GET HIGH 4 BITS
MOV BX, AX ; SAVE ROTATED VALUE
AND AX, 0FFF0H ; GET SHIFTED LOW 4 NIBBLES
ADD
AX, OFFSET RECV_BUFF
; NOW LOW BYTES OF POINTER ARE IN AX.
ADC BX, 0 ; ADD IN THE CARRY
; TO THE HIGH NIBBLE
AND BX, 000FH ; GET JUST THE HIGH NIBBLE
MOV DX, D3DSTL
OUT DX, AX ; AX=LOW 4 BYTES
MOV DX, D3DSTH
MOV AX, BX ; GET HIGH NIBBLE
OUT DX, AX
; DESTINATION POINTER DONE. SOURCE IS IN PCB.
MOV DX, D3SRCL
MOV AX, S1RBUF ; RECEIVE BUFFER FOR
OUT DX, AX ; CHANNEL 1 IS DEST
Example 10-2. DMA-Driven Serial Transfers (Continued)
10-35