Although I enjoy all of the content on the PageTable blog, I like the archeology stuff the most. This post about the hack used to do directory listings on the Commodore is a great example.
Essentially, the Commodore's disk drive acted as a co-processor
handling all of the mass storage management and file system
operations. The Commodore's kernel and BASIC shell used a file
oriented API (i.e. open, close, read, write) to communicate with the
floppy drive. Commands like list-directory were performed by an
open() call to a special address and file name and reading the
results from the open "file" until an EOF was received.
For reasons that are unclear from the post, it seems that the
Commodore's kernel and BASIC shell were unable to natively perform the
list-directory operation. As a result, a hack was needed to avoid
users from having to write a BASIC program every time they wanted to
list a directory.
The solution was for the floppy drive to return the directory listing
in the format of a BASIC program. Thus, a list-directory operation
consisted of:
- opening the special file
- reading the returned fake program into memory as a program
- listing the fake program to display the directory contents
What a clever hack!
If I understood the post correctly, the returned fake program simply
used the structural description of a BASIC program but wasn't a valid
program itself. In other words you couldn't execute the program
returned by the list-directory operation. Too bad because it would
have been really cool if the floppy drive returned a lambda function!
