[Coco] 6309 bitmd instruction

Gene Heskett gene.heskett at verizon.net
Sun Feb 19 16:32:28 EST 2006

On Sunday 19 February 2006 14:19, Boisy Pitre wrote:
>Does anyone know how the 6309 instruction BITMD affects the CC
>register?  In my debugging efforts, I am seeing ZERO change in the CC
>register when using this instruction, and am wondering how this
>instruction actually works.
>Any insight, anyone?

Chet Simpson, many years ago now, wrote a nomograph about the 'new' 6309 
codes.  It might be in there, but it would take me hours to lay my 
hands on the copy I printed out way back then.  Hey, I got lucky, it 
hadn't gone to the basement yet.  Only took about 2 minutes to dig it 
off the bookshelf over where the coco used to live.

From that, on page 3:
The MD register is a mode and error register and works much in the same 
way as the CC register.  The bit definitions are as follows:

Write bits
 Bit0 - Execution mode of the 6309
  If clear(0), the cpu is in the 6809 emulation mode
  If set(1), the cpu is in 6309 native mode
 Bit1 - FIRQ mode
  If clear(0), the FIRQ will occur normally
  If set(1), the FIRQ will operate the same as the IRQ

Bits2-5 are unused.

Read Bits - One of these is set when the 6309 traps an error
 Bit6 - This bit is set(1), if an illegal instruction is encountered
 Bit7 - This bit is set(1), if a zero division occurs

And thats all it says about the MD register, Boisy.  As for
BITMD - Test any bit or bits in the MD register.

Its opcode is $113C, its an immediate mode instruction, takeing 4 cycles 
to execute, and 3 bytes total in the execution input stream.  I'd say 
the third byte is the bit spec to read.  One of $40, $80 or $C0 
depending on which bit or bits you wanted to read.  I never found a use 
for it so I can't confirm thats how it works myself.  Also don't know 
if the write bits can be read back too.

Does this help?

Cheers, Gene
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