1st stage loader for IMSAI - need ascii to binary conversion

[Marden P. Marshall]

On Feb 25, 2008, at 9:58 AM, Jeff Erwin wrote:

> I am in the process of rebuilding an IMSAI 8080 and am using my Mac  
> Pro as a
> dumb terminal.  I have it all to the point where I can type a  
> character and
> see the ascii appear on the front panel.  What I need to do is write a
> simple 1st stage loader that will take the HEX output from the ASM80
> assembler, send it over the serial connection and put it in memory.   
> This
> way I can write a more sophisticated loader and not have to key it  
> in by
> hand.
>
> The question is, what is the algorithm, in assembly language, for  
> converting
> an ascii character to its binary equivalent.  By this, I mean  
> converting an
> 'a'  to 1010, not 041H which is the ascii value.  I remember writing  
> such a
> thing 30 years ago but simply cannot get my head around it now.
>
> The algorithm has to deal with taking an ascii string like
> AF67DBFF6FF9AF81C2130053DBFF5FAAC2210039D20600780747D3FFAF4 and  
> converting
> each ascii character into the binary value it represents.  The sting  
> above
> would convert, one character at a time, to:
>
> 1010
> 1111
> 0110
> 0111
> 1101
>
> well, you get the point...It has to work only for 0-9 and A-F which  
> should
> make it easier.
>
>
> Anyone out there remember how to do this?
>
> Jeff Erwin
>

Let me try this again.

I just went through this same exercise last week.  Here is a link to  
the source code of IMSAI's original paper tape loader which does  
exactly what you're looking for.  You will probably have to modify it  
to suit your particular serial interface and memory map.

http://maben.homeip.net/static/S100/IMSAI/code/1976%20Self%20Contained%20System/paper%20tape%20loader.asm

As an aside, here is a video of my IMSAI using this loader to bring up  
a copy of Altair 4K Basic.

http://www.youtube.com/watch?v=ADplHpk33yY


-Mardy