Posted By
SVS
on 2019-04-02
08:42:23
 |  Re: Puzzling about OPEN
Hi again,
I've investigated about the difference between B-R and U1.
The B-R (and mirrored the B-W) takes the first byte of the sector as number of valid bytes to be read. Then if a sector contains, for example, "03 41 51 6A F8 ..." it will transfer only 41 51 6A (3 bytes). This way can be useful for some customized data structures, anyway each sector allowing 1 less byte of data. The U1 on the contrary transfers the whole sector of data starting from the first byte.
About the matter of the ":". It is a flag for the DOS to indicate the end of the command. You can send the command as long as you want, it acknowledges only the first character (for ex. N can be NEW, NE, N, but even NEWAGE  ). When found the ":" it understands that the command name is finished and the following data (if any) are the parameters.
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