* * * * *

  I know languages that have support for “read-only memory,” but what about
                             “write-only memory?”

I'm still hacking away on my overengineered 6809 assembler [1] and one
feature I've beem mulling over is a form of static access checking. I have
byte-level access control when running tests [2] but I'm thinking that adding
some form of “assemble time checking” would also be good. I've been writing
code hitting the hardware of the Color Computer, and there are semantics
around hardware that I don't think many languages (or even assembler) support
write only memory! As the _Amiga Hardware Reference Manual_ [3] states (only
referenced here because it's a good example of what I'm talking about):

> Registers are either read-only or write-only. Reading a write-only register
> will trash the register. Writing a read-only register will cause unexpected
> results.
>
> …
>
> When strobing any register which responds to either a read or a write, (for
> example copjmp2) be sure to use a MOVE.W, not CLR.W. The CLR instruction
> causes a read and a clear (two access) on a 68000, but only a single access
> on 68020 processors. This will give different results on different
> processors.
>

The Color Computer isn't quite as finicky (although the 6809 inside it also
does the “read, then write” thing with the CLR instruction), but there is
still memory-mapped IO (Input/Output) that is read-only, some that is write-
only, and some that has different meanings when reading and writing. And
while C has some semantic support with volatile (to ensure reads and writes
happen when stated) and const (to ensure the read-only nature) it still lacks
a “write-only” concept. And I've never used an assembler that had “write-
only” semantics either.

I'm thinking something along these lines:

-----[ Assembly ]-----
               org     $FF40
DSK.CTRL        rmb/w   1       ; write only

               org     $FF48
DSK.CMD         rmb/w   1       ; write only
               org     $FF48
DSK.STATUS      rmb/r   1       ; read only

DSK.TRACK       rmb     1       ; these can be read and written
DSK.SECTOR      rmb     1
DSK.DATA        rmb     1
-----[ END OF LINE ]-----

Here, the RMB directive just reserves a number of bytes, with a default
access of “read-write.” The /W or /R designates that label as being either
“write-only” or “read-only.” And if you look closely, you'll see that both
DSK.CMD and DSK.STATUS are defined as the same address. It's just that
writing to that address will send a command to the drive controller, while
reading from that address give the current status. The only issue I have are
hardware registers that can be programmed for input or output. The MC6821 [4]
used in the Color Computer has such registers, and the issue I have is how to
signify this change in state in a program—that at this point in the program,
such-n-such address is “read-write” but afterwards, it's “write-only.”

[1] https://github.com/spc476/a09
[2] gopher://gopher.conman.org/0Phlog:2024/01/19.1
[3] https://www.amazon.com/exec/obidos/ASIN/0201181576/conmanlaborat-20
[4] https://en.wikipedia.org/wiki/Peripheral_Interface_Adapter
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