80C186EC/80C188EC Microprocessor User's Manual

DIRECT MEMORY ACCESS UNIT
$mod186
name
DMA_WITH_SCU
; The following example initializes the DMA unit to perform
; DMA-driven serial transfers.
;
; It is assumed that the serial port has been initialized for
; Mode 1 asynchronous transfers. Register mnemonics are assumed
; to be defined elsewhere in EQUate instructions.
DATA
SEGMENT
XMIT_BUFF DB 'This is a serial message.'
RECV_BUFF DB 128 DUP('ReCv') ; JUNK DATA
DATA
CODE
ENDS
SEGMENT
ASSUME CS:CODE
MOV AX, DATA ; DATA SEGMENT POINTER
MOV DS, AX
ASSUME DS:DATA
; First we set up DMA channel 2 (Module B, channel 0) to handle
; transmit requests from serial port 1.
;
; The source of data is the transmit buffer in memory.
; The destination for data is the TBUF register for serial port 1...
MOV
AX, SEG XMIT_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 XMIT_BUFF+1
; USE XMIT_BUFF+1 BECAUSE FIRST BYTE IS SENT MANUALLY.
; 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, D2SRCL
OUT DX, AX ; AX=LOW 4 BYTES
MOV DX, D2SRCH
MOV AX, BX ; GET HIGH NIBBLE
OUT DX, AX
; SOURCE POINTER DONE. DESTINATION IS IN PCB.
MOV DX, D2DSTL
MOV AX, S1TBUF ; TRANSMIT BUFFER FOR
OUT DX, AX ; CHANNEL 1 IS DEST
Example 10-2. DMA-Driven Serial Transfers
10-34