From: james.harris.1@nospicedham.gmail.com   
      
   On 18/07/2017 09:59, wolfgang kern wrote:   
   >   
   > James Harris asked:   
   >   
   >> I wrote:   
   >>> Martin Str|mberg wrote:   
   >>> ...   
   >>>> So you decide the rules and tell and show us (if you want).   
   >>>   
   >>> Thanks, I thought real needs caused this restrictions.   
   >   
   >> As someone who has seen the various attempts at writing a decoder I am   
   >> not sure what the goal is. I get it that you have been producing code   
   >> which uses bytes in the ASCII printable character set but there are   
   >> lots of questions.   
   >>   
   >> What is the basic requirement?   
   >   
   > the challenge was/is to create the shortest possible BASE64 decoder with   
   > transferable text to be renamed as a xxx.COM and be executed in DOS.   
      
   Thanks for the explanation, and those below. Weeks of mystery are now   
   resolved. :-)   
      
      
   >   
   >> What are the initial conditions, i.e. what do you know about registers   
   >> at entry?   
   >   
   > know register in DOS.com files are SS=DS=ES=CS SP=FFFE or less [SP]=0.   
      
   OK.   
      
      
   >> What's the minimum number of bytes you can count on before you might   
   >> get to a CR or CR/LF?   
   >   
   > I think we agreed on 64-characters/line but it can be more or less or   
   > even reflown during transfer. So we got to skip any space, tabs CR/LF.   
      
   Skipping later CR or CR-LF is one thing but I was thinking that having   
   either of them potentially inserted in the early bytes of the decoder   
   code would be very tricky.   
      
      
   >> Can you overwrite the original text with the decoded version?   
   > yes.   
      
   OK. Good!   
      
      
   >   
   >> If not, what options can be used to get memory to decode into?   
   >   
   > nothing, DOS.com is limited to ~63.5 KB. I use PSP CS:040.. for decoder.   
   >> In addition, I could guess there might be problems with the location   
   >> of the decoded code. So I guess it is meant to be started at CS:0 or   
   >> CS:100. Am I anywhere close?   
   >   
   > The challenge implied that the final decoded stuff starts at CS:0100 and   
   > run (or as additional challenge to be stored) as a .com-file.   
      
   OK. I have to say that that raises challenging issues of alignment if   
   the decoded output is to overwrite the input.   
      
      
   >>> So my rules for such an Base64 decoder would allow ASCII 33..127 and   
   >>> nothing less than 386 code.   
   >   
   >> You know you want to use 8086 code really.... :-)   
   >   
   > I'm much older than 8080, but I wont enter the museum to run my code.   
      
   I remember programming the 6502 and the Z80. Happy days! But I never   
   went back as far as the 8080.   
      
      
   --   
   James Harris   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)