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Rhad
PostPosted: Tue Nov 05, 2019 11:32 pm 
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Posts: 2
Hi all, I'm trying to work out the bugs in a 6502 emulator I'm writing so I'm going through Dormann's testing suite and I'm stuck on this test. It zeroes out all the flags, goes through a bunch of tests to make sure that branch instructions work properly, and then checks the flags. For some reason, it's expecting the break and reserve flags to be set, but I can't see what would set them? Any help here would be greatly appreciated, I must be missing something.

Code:
                                set_stat 0      ;all off
                       >            load_flag 0     
0644 : a900            >            lda #0                   ;allow test to change I-flag (no mask)
                       >
0646 : 48              >            pha         ;use stack to load status
0647 : 28              >            plp

                                           //Skipped a bunch of branch tests here, only branch instructions, none of which affect flags

0670 : 08               br14    php
0671 : 68                       pla
                                cmp_flag 0      ;flags off except break (pushed by sw) + reserved?
0672 : c930            >            cmp #(0      |fao)&m8    ;expected flags + always on bits
                       
                                trap_ne
0674 : d0fe            >        bne *           ;failed not equal (non zero)
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Dr Jefyll
PostPosted: Wed Nov 06, 2019 12:40 am 
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Joined: Fri Dec 11, 2009 3:50 pm
Posts: 3367
Location: Ontario, Canada
Hello, Rhad, and welcome!

Until now I've never looked at any of the code in Klaus's suite, but based on this snippet there does seem to be a puzzle here.

Is it possible the following lines are confusing your assembler? (I know they're confusing me! :wink: ) Anyway, maybe the result is something other than what Klaus's assembler would give. For me that's the first thing that comes to mind -- an assembler issue (with the expression for the CMP instruction).
Code:
                                cmp_flag 0      ;flags off except break (pushed by sw) + reserved?
0672 : c930            >            cmp #(0      |fao)&m8    ;expected flags + always on bits

But the comment does say, ""except break (pushed by sw) + reserved"... followed by a question mark. Maybe someone will pipe up who can explain that.

Edit: I've gotten too distracted by "sw" and the odd expression, and have overlooked the actual issue :oops: -- which Chromatix identifies below.

cheers
Jeff

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Last edited by Dr Jefyll on Wed Nov 06, 2019 2:14 pm, edited 1 time in total.

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Chromatix
PostPosted: Wed Nov 06, 2019 1:28 am 
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Joined: Mon May 21, 2018 8:09 pm
Posts: 1462
In the 6502, only six of the bits in the status register actually exist in hardware - N, Z, C, V, I, and D (not in that order). The other two always read as set bits when pushed with PHP.

The B bit is purposefully manipulated when the status register is pushed on the stack by an interrupt, to indicate to the interrupt handler routine whether it was a hardware interrupt or the BRK instruction that triggered it. The "reserved" bit remains set even in this case.
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Rhad
PostPosted: Wed Nov 06, 2019 3:39 am 
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Joined: Tue Nov 05, 2019 11:11 pm
Posts: 2
Wow. After poring back through the programming manual trying to find the documentation for this behavior I found out I somehow chose to start skimming at the very paragraph where they talk about this in the interrupt chapter. Thanks, probably never would've figured that out on my own.
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BigEd
PostPosted: Wed Nov 06, 2019 9:13 am 
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Joined: Thu Dec 11, 2008 1:28 pm
Posts: 10986
Location: England
(Welcome, Rhad! Hope to hear more about your emulator in due course. The behaviour of B often trips up emulators. I think there are very few emulators indeed which pass Klaus' test suite first time they encounter it.)
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