[Coco] Assembly help: Corrupted bin file ?

[Robert Gault]

I'll add to what Art and William wrote about multiple ORG statements.

The term "multi-segment source file" is somewhat deceptive. The precise 
meaning relates not to the source code at all but the manner in which 
the compiled file is stored on disk.
The Coco does not store bare binary programs on disk. The only way to 
create these file with just a Coco is to create a data file. The Coco 
always has at least a preamble and postamble defining where the program 
loads into memory, how long it is, and where the execution address is 
located.
If the only the initial preamble and postamble are present, then the 
file is a single segment file. This type of storage is not the default 
for the Tandy EDTASM program. It defaults to multi-segment files that 
typically have segments of 256 bytes. Each segment has a preamble. This 
happens even if the is a single ORG statement in the source code.
If there are multiple ORG statements, then each ORG could engender a 
preamble in the stored file.

So, how does the above apply to EDTASM or RainbowIDE? EDTASM has an 
assembly option to force the single segment format if there is no more 
than one ORG statement. A "FILENAME" /SR   which stands for single 
record. RainbowIDE offers many more options in the Output Object 
drop-down entry.

The net result is that the number of ORG statements and assembler 
options determine how or if the resulting binary is segmented.

===============
You already got from another reader the answer to controlling the the 
start of the PMODE screens. To restate it, bytes $FFC6-$FFD3 do this. 
Each even byte is a 0, each odd byte a 1. The bits are combined into a 
seven bit word that points to the start of the video address for a 
Coco1, Coco2, or Coco3 in Coco1 mode. $FFC6/$FFC7 is the least 
significant bit and $FFD2/$FFD3 the most significant bit.
Screen start / by $200 = video offset
Low res text screen is $400/$200=2=%10  = $FFC9 $FFC6
Beginning of the PMODE screen on a non-disk system
  $600/$200=3=%11 = $FFC9 $FFC7
Beginning of the PMODE screen on a disk system
  $E00/$200=7=%111 = $FFCB $FFC9 $FFC7