PORTS

The 80xxx family communicates with hardware devices by
means of PORTS

A PORT is an ADDRESS where an I/O device is connected
Data is SENT TO the device by sending it to the port
Data is received/INPUT FROM the device by reading it
from the port

A PORT ADDRESS can be any value between 
0000H and FFFFH

Instructions for dealing with ports:

	IN	Accum, Immed8

Accum MUST be AL or AX
If AL then a byte is transferred
IF AX then a word is transferred

Transfer data FROM 8-bit PORT address given by Immed8
operand TO Accum. For ports  between 00H and FFH

	IN	Accum, dx
Transfer data FROM 16-bit PORT address given in DX
operand TO Accum. This is faster if we ignore time to
load DX.
	OUT	Immed8, Accum
	OUT	dx, Accum
Transfer data TO PORT address given in Immed8/DX
operand FROM Accum. 

Before we start moving things to/from ports we need to
set it up. We  need to know about the interface chip to
do this.

The Lab boards have two 8255A-5 Peripheral Interface
Adapters (interface chip) from Intel. 

One chip (the BOARD 8255) is connected to the LEDs,
Speaker, keypad, etc. 

The other (the USER 8255) is for the user to use for
custom projects.

The Board 8255 is divided into 3 8-bit  ports.

PORT A: LEDS
PORT B: later
PORT C: Keypad

Before you use the CHIP to flash LEDS, you need to tell
it that PORT A is an output port, Port B is also output
and Port C is (half) input.

See figure  1-1 in Interface Manual.

Each PORT on the Board's 8255 is either INPUT or
OUTPUT with no handshaking (never mind what
handshaking is :-)).

This is MODE 0 for the 8255

According to fig 1-1 we want to tell the chip that
we want:

PORT A to be OUTPUT
PORT B to be OUTPUT
Lower Nibble (LSN) of PORT C to be OUTPUT
Upper Nibble (MSN) of PORT C to be INPUT 

Look at  INTEL 8085 data sheets
Appendix D in the Interface Manual

To tell the chip we want this configuration
We send the control byte   88h to port 303h

	mov	DX, 303h
	mov	AL, 88H
	OUT	DX, AL

We could also use TVC to output 88h to port 303h
You need to get into privelage mode first
then
>> priv
-- O 303 88
--
should cause all LEDS to light up

Why Port 303?

The board is set up that way.
In hex:
	300	Port A
	301	Port B
	302	Port C
	303	Mode Port

Sending the Mode control Byte to the mode port
sets up the chip the way we want it.

How do we make the chip flash the LEDS

Sending a Byte to PORT A (300h) will cause the 
LEDS specified by the PORT to Light up.

Bit 0 	corresponds to 	Rightmost LED
.
.
.
Bit 7	corresponds to 	Leftmost LED

The LEDS are connected so that a logic  0 turns them on
while a Logic 1 turns them off.

-- O 300 FF

will turn off all LEDS

-- O 300 FE will turn on only the first LED

TITLE   flash1.asm 
 
DOSSEG 
.MODEL  SMALL 
 
.STACK  100h 
 
.DATA    
	 
.CODE 
 
START: 
	mov     ax, @DATA 
	mov     ds, ax 
 
	mov     dx, 303h 
	mov     al, 88h 
	out     dx, al 
	 
	mov     al,-1 
	mov     dx, 300h 
	out     dx, al 
 
	mov     al, -1 
	mov     bl, 1 
 
TOP: 
	xor     al, bl 
	out     dx, al 


	 
	mov     cx, 0 
l1:           loop    l1 	
			; A delay Loop, loop doing nothing 
 			; for a while


	mov     cx, 0   
l2:           loop    l2      
			; Without Delays, the LEDS flash 
			; too fast 
			; Just kinda flicker on and off
 
	shl     bl,1 	

	; shift the contents of bl left by 1
	; shl 	Op1, 1  shift left by 1
	; shl	Op1, cl shift left by count in cl
	; must use cl no other register
 
	jnz     TOP 
 
	CALL    EXIT 
 
END     START 

TITLE   as4.asm
...

EXTRN  EXIT:NEAR

START:
	mov     ax, @DATA
	mov     ds, ax

	CALL	INIT

	....

CNTR:

	CALL	DELAY

	CALL	LEDOFF

	inc 	bl

	CALL	LEDCNT

	loop	CNTR	

	CALL	EXIT


;----------------------------------------------

PROC	LEDCNT		NEAR

	push	ax 	; Save AX
	push 	dx	; Save DX

	...

	pop	dx
	pop	ax

LEDCNT		ENDP

	...; other procedures...

END	START