[Coco] 6309 bitmd instruction

[jdaggett at gate.net]

One has to remember that the BIT instruction of the 6809/6309 
does a logical "AND" with the register and the constents of memory 
or a mask bit. Depending on what you are anding the MD register 
with will determine the state of the Z and N bits of the CCR. The 
Carry and Half Carry bits are unaffected and the Overflow bit is set 
to zero.

james


On 19 Feb 2006 at 16:32, Gene Heskett wrote:

> 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
> People having trouble with vz bouncing email to me should add the word
> 'online' between the 'verizon', and the dot which bypasses vz's
> stupid bounce rules.  I do use spamassassin too. :-)
> Yahoo.com and AOL/TW attorneys please note, additions to the above
> message by Gene Heskett are:
> Copyright 2006 by Maurice Eugene Heskett, all rights reserved.
> 
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