Date   : Thu, 28 Feb 2008 10:39:57 +1100
From   : msmcdoug@... (Mark McDougall)
Subject: BBC FPGA and B flags

Hi again,

Have corresponded with the 6502 core maintainer and we are currently 
deciding how the B flag should operate. Pending this discussion he'll do a 
fix and advise - hopefully within the next 24 hrs! :)

According to the MAME documentation, the (NMOS) 6502 *always* has the B flag 
set in the P register. According to the code itself, it is set on reset, and 
*never* set to 0 there-after. As is commonly accepted, in response to an 
IRQ/NMI, the processor pushes P onto the stack with the B flag set to zero - 
P.B is otherwise unaffected. Likewise in the MAME 6502 core, PLP and RTI 
instructions both (also) set P.B. FWIW BRK pushes P with B flag set.

Regards,

-- 
|              Mark McDougall                | "Electrical Engineers do it
|  <http://members.iinet.net.au/~msmcdoug>   |   with less resistance!"