Some helps in register Window of VICE Monitor.

nbla000 · Post by **nbla000** » Wed Feb 07, 2007 11:11 am

Using the register windows in VICE MONITOR (View --> Register window) for debugging purpose, i don't understand some bytes and flag as show in image below.
Someone know the meaning of bytes SP, 00, 01, FL and flags - and B ?

Schlowski · Post by **Schlowski** » Wed Feb 07, 2007 12:32 pm

SP is stack pointer, 00 and 01 are the contents of memory bytes 0 and 1 (especially useful on C-64, not on VIC). B is break-flag (set after BRK opcode) and - is unused.

carlsson · Post by **carlsson** » Wed Feb 07, 2007 5:39 pm

And in case you don't know where 6502 stores its stack, it is page 1 ($0100-$01FF) counting from top to bottom. SP=F3 means the stack pointer is at $01F3. If you pop it (PLA, PLP) it will become F4. If you instead would push (PHA, PHP) something onto it, it is at F2. TSX transfers SP to X register. TXS does the opposite.

nbla000 · Post by **nbla000** » Thu Feb 08, 2007 2:33 am

Thanks guys

OPS, the FL value what does mean ? and why if - flag is not used sometime is 1 and other 0 ?

MacbthPSW · Post by **MacbthPSW** » Thu Feb 08, 2007 5:56 pm

nbla000 wrote:OPS, the FL value what does mean ? and why if - flag is not used sometime is 1 and other 0 ?

FL is just the hex representation of the flag register... %00100010 in your example is $22. Kind of redundant, but I guess somebody might care.

The unused flag is called the "expansion bit". According to one book of mine "It is most likely that this bit will appear to be on when one is analyzing the bit pattern in the processor status register; however, no guarantee as to its state is made as this bit will be used in expanded versions of the microprocessor".

nbla000 · Post by **nbla000** » Mon Feb 12, 2007 12:03 pm

MacbthPSW wrote: FL is just the hex representation of the flag register... %00100010 in your example is $22.

ops..