Intel 80C186XL, 80C188XL user manual Direct Memory Access Unit, Example 10-2.Timed DMA Transfers

DIRECT MEMORY ACCESS UNIT

$mod186

name DMA_EXAMPLE_1

;This example sets up the DMA Unit to perform a transfer from memory to

;I/O space every 22 uS. The data is sent to an A/D converter.

;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, set up the pointers. The source is in memory.

	MOV	AX, SEG WAVEFORM_DATA
	ROL	AX, 4	; GET HIGH 4 BITS
	MOV	BX, AX	; SAVE ROTATED VALUE
	AND	AX, 0FFF0H	; GET SHIFTED LOW 4 NIBBLES
	ADD	AX, OFFSET WAVEFORM_DATA
; 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, AX	; AX=LOW 4 BYTES
	MOV	DX, D0SRCH
	MOV	AX, BX	; GET HIGH NIBBLE
	OUT	DX, AX
	MOV	AX, DA_CNVTR	; I/O ADDRESS OF D/A
	MOV	DX, D0DSTL
	OUT	DX, AX
	MOV	DX, D0DSTH
	XOR	AX, AX	; CLEAR HIGH NIBBLE
	OUT	DX, AX
; THE POINTER ADDRESSES HAVE BEEN SET UP. NOW WE SET UP THE TRANSFER COUNT.
	MOV	AX, 255	; 8-BIT D/A, SO WE SEND 256 BYTES
	MOV	DX, D0TC	; TO GET A FULL SCALE
	OUT	DX, AX
; PROGRAM	IDRQ MUX
	MOV	DX, DMAPRI
	MOV	AX, 00H	; TIMER2 IS IDRQ SOURCE
	OUT	DX, AX	; MODULES HAVE EQUAL PRIORITY

Example 10-2. Timed DMA Transfers

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