(My original message to cctech has yet to appear. I thought I might try the
cctalk list).
While Motorola never shipped the MC6839 [1] the binary is available [2]
and I've been playing around with it [3]. While it's not producing the
exact same results as I get on a more modern machine, it appears to be
"close enough" for me to be happy with it. But I am having one issue that I
can't figure out.
The documentation for the FMOV operation says:
FMOV Move (or convert) arg1 -> arg2. This function is useful for
changing precisions (e.g. single to double) with full
exception processing for possible overflow and underflow.
Okay. And to call it [4]:
------------------------------------------------------------------------------
|Function|Opcode| Register entry conditions | Stack entry conditions
------------------------------------------------------------------------------
| FMOV | $1A | U = precision parameter word| push arg
| | | Y -> argument | push precision param
word
| | | D -> fpcb | push ptr to fpcb
| | | X -> result | call FPO9
| | | | pull result
------------------------------------------------------------------------------
For moves, U contains a parameter word describing the size of the
source and destination arguments. The bits are as follows, where
the size is as defined in the fpcb control byte
Bits 0-2 : Destination size
Bits 3-7 : unused
Bits 8-10 : Source size
Bits 11-15: unused
And the size bits are defined as:
111 = reserved
110 = reserved
101 = reserved
100 = extended - round result to double
011 = extended - round result to single
010 = extended - no forced rounding
001 = double
000 = single
It appears that to convert from single to double, I would set U to $0001,
but the results are *so* far out of whack it's not even funny. I've tried
setting U to point to the value $0001 and that doesn't work. I've tried
shifting the bits (because in the FPCB they're the upper three bits) and
that doesn't work. I've tried reversing the registers and that doesn't
work. Does anyone have the actual source code [4]? Or know what I might be
doing wrong?
-spc
[1] A ROM with position independent 6809 object code that conforms (to
what I can find) with IEEE 754 Draft 8.
[2] Available in the file fpo9.lzh here
https://ftplike.com/browser/os9archive.rtsi.com/OS9/OS9_6X09/PROG/
[3] Using a 6809 emulator library I wrote: https://github.com/spc476/mc6809
Not much documentation I'm afraid.
[4] Register entry: ROM base address + $003D
Stack entry: ROM base address + $003F
[5] I'm lead to believe that Motorola release the code into the public
domain.
