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6502 relative Branch = x86 jump short ?
Posted: Fri Jul 07, 2017 11:43 pm
by Sailor
hi every body
I am trying to simulate each instruction of the 6502 in C ( open watcom v1.8 )
by using inline assembly , my question is that :
are the 6502 branch instructions equal (the same) to the X86 jump short instructions ?
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 12:26 am
by GARTHWILSON
Hopefully someone will give a better explanation comparing to x86; but the operand on 6502 relative-branch instructions is an offset
relative to the first byte of the next instruction, meaning for example D0 00 (ie, BNE next_instruction) is a branch with no effect. An operand with the high bit set is to branch backwards; so D0 FE will conditionally branch back to itself.
No 6502 / 65c02 / 65816 programmer should be without the programmer's manual "
Programming the 65816—Including the 6502, 65C02 and 65802" by David Eyes and Ron Lichty. It's way better than any other I know, and way better than the description lets on.
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 9:35 am
by Sailor
OK
then how to implement 6502 branching in C (if not in inline assembly) ?
is the following code do the job ? :-
Code: Select all
char addr ; //address of branching , char range is -128 to 127
unsigned int PC ; // word Program Counter
PC+=addr // if address is <=127 branching is forward , if address >127 branching is backward
// add 2 to addr ? because branching instructions are 2 bytes
your suggestions please .
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 11:17 am
by Martin A
It would appear 8086 short jump and the 6502 branching instructions calculate their offsets the same way.
Using the build in assembler in BBC basic, I've produced 2 simple bits of code:
BBC Basic for DOS Produced:
Code: Select all
8086 Branching
099F OPT X%
099F B0 00 MOV AL,0
09A1 .branch
09A1 FE C0 INC AL
09A3 75 FC JNE branch
09A5 C3 RET
While BBC Basic on the 6502 produced:
Code: Select all
6502 Branching
08B4 OPT X%
08B4 A9 00 LDA #0
08B6 .branch
08B6 69 01 ADC #1
08B8 D0 FC BNE branch
08BA 60 RTS
The 4 byte backward branch on both CPUs usees an offset of &FC
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 1:05 pm
by dwight
I don't see anyone mentioning page crossing
behavior?
Dwight
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 1:49 pm
by Sailor
It would appear 8086 short jump and the 6502 branching instructions calculate their offsets the same way.
Using the build in assembler in BBC basic, I've produced 2 simple bits of code:
BBC Basic for DOS Produced:
Code: Select all
8086 Branching
099F OPT X%
099F B0 00 MOV AL,0
09A1 .branch
09A1 FE C0 INC AL
09A3 75 FC JNE branch
09A5 C3 RET
While BBC Basic on the 6502 produced:
Code: Select all
6502 Branching
08B4 OPT X%
08B4 A9 00 LDA #0
08B6 .branch
08B6 69 01 ADC #1
08B8 D0 FC BNE branch
08BA 60 RTS
The 4 byte backward branch on both CPUs usees an offset of &FC
that is what I have expected , so could I depend on that and continue ?
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 6:33 pm
by GARTHWILSON
I don't see anyone mentioning page-crossing behavior?
Crossing a page boundary adds one cycle to the branch, from three cycles to four, for 65(c)02 and for the 65816 running in emulation mode. The 65816 running in native mode doesn't need the fourth clock. Regardless, most branches do not cross a page boundary.
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 8:26 pm
by dwight
I don't see anyone mentioning page-crossing behavior?
Crossing a page boundary adds one cycle to the branch, from three cycles to four, for 65(c)02 and for the 65816 running in emulation mode. The 65816 running in native mode doesn't need the fourth clock. Regardless, most branches do not cross a page boundary.
I was talking about when the instruction it self crosses an page boundary. I recall that it had a similar
problem the the NMOS parts had for the indirect jump instruction where it didn't see that the high part
of the PC had changed and didn't do a correct page crossing.
I could be wrong because it has been many years. It may only be the indirect jump that had the problem.
Dwight
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 8:38 pm
by GARTHWILSON
That was on JMP(xxFF). NMOS did not increment the page when reading the second byte, so the jump was wrong. That bug (and all others) were fixed in the CMOS version. There was no bug in the branch instructions though. I rounded up all the NMOS-CMOS differences I could find and put them on the page
http://wilsonminesco.com/NMOS-CMOSdif/ .
Re: 6502 relative Branch = x86 jump short ?
Posted: Sat Jul 08, 2017 9:05 pm
by dwight
That was on JMP(xxFF). NMOS did not increment the page when reading the second byte, so the jump was wrong. That bug (and all others) were fixed in the CMOS version. There was no bug in the branch instructions though. I rounded up all the NMOS-CMOS differences I could find and put them on the page
http://wilsonminesco.com/NMOS-CMOSdif/ .
I was in error this only effects the indirect jump and not the branch.
I don't know why I thought this.
I just ran the experiment so I'm sure I was in error.
Dwight
Re: 6502 relative Branch = x86 jump short ?
Posted: Sun Jul 09, 2017 4:39 pm
by Sailor
OK
then how to implement 6502 branching in C (if not in inline assembly) ?
is the following code do the job ? :-
Code: Select all
char addr ; //address of branching , char range is -128 to 127
unsigned int PC ; // word Program Counter
PC+=addr // if address is <=127 branching is forward , if address >127 branching is backward
// add 2 to addr ? because branching instructions are 2 bytes
.
Re: 6502 relative Branch = x86 jump short ?
Posted: Sun Jul 09, 2017 5:30 pm
by dwight
In a simulator, it might be better to increment the PC after each
instruction fetch. That way, all you have to do is add the branch offset as
a signed byte to the PC. If bit 8 changes in the PC add the
extra cycle to your cycle counter.
All this within C.
Oops, I just reread your original post. Are you simulating from
the text or the binary image after the assembly?
Dwight
Re: 6502 relative Branch = x86 jump short ?
Posted: Sun Jul 09, 2017 6:31 pm
by Sailor
Dwight
thanks for reply , all reading done as binary .
Re: 6502 relative Branch = x86 jump short ?
Posted: Mon Jul 10, 2017 5:29 am
by dwight
Then what I suggested should work fine.
I do hope you have a cycle counter. You also want an
easy way to connect to it with user code. Outside interaction
is the most important part of a simulator.
With the cycle count, you can tack on things like a usart, video
action or even a random input timed keyboard. Without it
it just runs instructions and little else.
Dwight
Re: 6502 relative Branch = x86 jump short ?
Posted: Mon Jul 10, 2017 6:13 pm
by Sailor
OK , but I have no idea how to count cycles .