Intel 80C188EC User Manual page 320

$MOD186
name DMA_EXAMPLE_1
; This example shows code necessary to set up two DMA channels.
; One channel performs an unsynchronized transfer from memory to memory.
; The second channel is used by a hard disk controller located in
; I/O space.
; It is assumed that the constants for PCB register addresses are
; defined elsewhere with EQUates.
CODE_SEG SEGMENT
ASSUME CS:CODE_SEG
START: MOV AX, DATA_SEG
MOV DS, AX
ASSUME DS:DATA_SEG
; First we must initialize DMA channel 0. DMA0 will perform an
; unsynchronized transfer from SOURCE_DATA_1 to DEST_DATA_1.
; The first step is to calculate the proper values for the
; source and destination pointers.
MOV AX, SEG SOURCE_DATA_1
ROL AX, 4
MOV BX, AX
AND AX, 0FFF0H
ADD AX, OFFSET SOURCE_DATA_1
; NOW LOW BYTES OF POINTER ARE IN AX
ADC BX, 0
AND BX, 000FH
MOV DX, D0SRCL
OUT DX, AL
MOV DX, D0SRCH
MOV AX, BX
OUT DX, AX
; SOURCE POINTER DONE. REPEAT FOR DESTINATION.
MOV AX, SEG DEST_DATA_1
ROL AX, 4
MOV BX, AX
AND AX, 0FFF0H
ADD AX, OFFSET DEST_DATA_1
; NOW LOW BYTES OF POINTER ARE IN AX
ADC BX, 0
AND BX, 000FH
MOV DX, D0DSTL
OUT DX, AX
Example 10-1. Initializing the DMA Unit
DIRECT MEMORY ACCESS UNIT
; DATA SEGMENT POINTER
; GET HIGH 4 BITS
; SAVE ROTATED VALUE
; GET SHIFTED LOW 4 NIBBLES
; ADD IN THE CARRY
; TO THE HIGH NIBBLE
; GET JUST THE HIGH NIBBLE
; AX=LOW 4 BYTES
; GET HIGH NIBBLE
; GET HIGH 4 BITS
; SAVE ROTATED VALUE
; GET SHIFTED LOW 4 NIBBLES
; ADD IN THE CARRY
; TO THE HIGH NIBBLE
; GET JUST THE HIGH NIBBLE
; AX=LOW 4 BYTES
10-31