Direct serial port read/write

[demolition]

I wanted to try to control the Amiga from the PC (to transfer small programs and run them etc.) so I thought it could be fun and relatively simple to do this through the serial port.

I found an example here but I have a few questions about the code, although it is very simple, but I'm sure the knowledgable people here can get me started with it. The author didn't bother to put any comments in it.

Here it is, with my comments added:

;routine for writing a single byte to the serial port without system calls.
  s: 	moveq		#0,d0                   ;
 	lea		data,a0                 ; a0 is now pointer to data
 	moveq		#6,d2                   ; number of bytes to send
 	move.w		#%0000001000000000,d3   ; Set stopbit
  writeit:
 	clr.b		d0		        ;
 	move.w		d3,d0                   ; Prepare d0 by setting stopbit
 	move.b		(a0)+,d0                ; Copy byte from data block and update pointer to next byte
 	move.b		d0,$dff030	        ; Put data in serial output buffer
 	clr.b		d1		       ; Why clear this?
 	move.b		$dff018,d1              ; Get data from serial input buffer
 	rol		#3,d1                  ;
 	btst		#1,d1                  ; Couldn't these two lines be replaced with 'btst #14,d1'?
 	bne		wait
  	cmp		#0,d2                   ; Are we done with all the data?
 	dbne		d2,writeit              ; Nah, go jump
 	rts                                     ; Ok, we are done
  	 
wait: 	btst	#1,d1                           ; Will this value ever change in the loop?
 	bne	wait                            ; Potentially wait forever?
 	beq	writeit
  
data: 	dc.b	'henrik'

Seems straightforward, but I have a few questions that I put in the comments. Any thoughts on this? I don't want to copy code, just understand it so I can make my own.

Edit: Looking at it again, I can see I first missed that he's only reading a byte from $dff018 and that bits 8+ in d1 is only written to with the rol instruction, although I'm not sure why he does this.

[Photon]

I like the data in the source there!

[prowler]

Yes, I imagine you would.

[demolition]

It's not your source by any chance (seeing as it's also from a swedish site)?

[Photon]

Hehe, no. I made a pretty fast parallel transfer routine from Amiga to C64 at one point.

BTW, this is just one end. Why not write how you set up the other end as well?

Actually, you're doing it the wrong way round! The Amiga should of course control the PC!

Not as silly as it sounds, I've often thought about writing a small program with a simple set of commands to give the Amiga the ability to make HTTP-requests and get data back, so that it would be able to access capable online resources and databases.

But it seems so much better to not need the PC. Still expecting ethernet stack+http that works on all Amigas I guess. Anyway, just dreamin'. All Amigas still couldn't handle surfing or streaming, but a command like 'Wikisearch <keyword>' or 'highscore.com/update.php?game=PunisherPenguin&handle=Photon&score=1gajillion' would be very low in requirements and data transferred, but still empower Amigas in a big way.

[demolition]

Well, the other end will be a PC running W7 with a USB-RS232 interface, so no asm there..

My idea was to put a program into the bootblock of a floppy which made it possible for me to transfer data from the PC to any address in the Amiga, read out data, running software (by making a JSR to a given address) etc. If the program you tested would crash, you just reset the Amiga and it would be ready to take orders again after a few seconds.
Combined with an automated upload from the PC, this would make it easy for me to quickly test small pieces of code on a real Amiga instead of in WinUAE which is not always accurate.
Actually, when I first tried how to use the copper on the Amiga, I could see the color changes in WinUAE but on the real Amiga the screen remained black. With an automated transfer setup, this would be much easier to get right.

[Thorham]

This code seems like utter nonsense to me. Do yourself a favor, and find some good code.