Intel 80C188XL, 80C186XL Direct Memory Access Unit, Example 10-1.Initializing the DMA Unit

DIRECT MEMORY ACCESS UNIT

$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	; DATA SEGMENT POINTER
	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	; GET HIGH 4 BITS
MOV	BX, AX	; SAVE ROTATED VALUE
AND	AX, 0FFF0H	; GET SHIFTED LOW 4 NIBBLES
ADD	AX, OFFSET SOURCE_DATA_1
; 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, D0SRCL
OUT	DX, AL	; AX=LOW 4 BYTES
MOV	DX, D0SRCH
MOV	AX, BX	; GET HIGH NIBBLE
OUT	DX, AX
; SOURCE POINTER DONE. REPEAT FOR DESTINATION.
MOV	AX, SEG DEST_DATA_1
ROL	AX, 4	; GET HIGH 4 BITS
MOV	BX, AX	; SAVE ROTATED VALUE
AND	AX, 0FFF0H	; GET SHIFTED LOW 4 NIBBLES
ADD	AX, OFFSET DEST_DATA_1
; 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, D0DSTL
OUT	DX, AX	; AX=LOW 4 BYTES

Example 10-1. Initializing the DMA Unit

10-23