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Forums > C64 Coding > bit zp,x TRUE or FALSE?

2012-07-02 18:53

Glasnost
Account closed

Registered: Aug 2011
Posts: 26

bit zp,x TRUE or FALSE?

I currently have an issue with some divergence in the 6510 opcodes.

Its opcode $34, which by various sources are noted to be the
opcode for..

bit zeropage,x

while vice and others mention it to be

nop zeropage,x

I unfortunately dont have a c64 anymore (boooohhh), so i even cant test it on the real thing.

Can anyone clarify this?

Best regards
Glasnost/Camelot

2012-07-02 19:22

HCL

Registered: Feb 2003
Posts: 717

Tested on real thing and it returns equal result as Vice (nop zp,x).

2012-07-02 19:37

Glasnost
Account closed

Registered: Aug 2011
Posts: 26

thx :)

2012-07-02 19:37

Cruzer

Registered: Dec 2001
Posts: 1048

The definitive opcode guide says nop too. Who are the lamers claiming it's bit?

2012-07-03 09:24

Slammer

Registered: Feb 2004
Posts: 416

I'm afraid the definitive opcode guide isn't stable. I still have the version from around 2005-2006 that shows four opcodes for bit ($24,$34,$2C and $3C)

Edit: I heard that the 6502C have the extra bit commands. Can anybody confirm this. According to the cpu listing in the opcode table page, these where used in the Atari computers and thats could be the reason for the change, Graham?

CPU versions:
-------------
6502: NMOS, used in Commodore disk drives, PET, various other 8 bit computers
6502C: 6502 with additional HALT pin, used in Atari 8 bit computer range
6510: 6502 with additional processor port, used in C64
8500: CMOS version of the 6510, used in C64C and C64G
8502: 2 MHz version of the 8500, used in C128
7501: HMOS-1 version of the 6502, used in C16/C116/Plus4
8501: HMOS-2 version of the 6502, used in C16/C116/Plus4

2012-07-03 13:47

HCL

Registered: Feb 2003
Posts: 717

Oh guys, uuhm.. I have to admit that i didn't actually open the box and check what kind of chip i has. It's the old box though..

2012-07-03 14:16

Skate

Registered: Jul 2003
Posts: 492

there is no such thing as "bit zp,x" in 6502 including plain 6502 used at Atari and other machines. "bit zp,x" is 65816 chip extension. so $34 is "nop zp,x" for 6502/6510 and "bit zp,x" for 65816.

2012-07-04 12:44

Flavioweb

Registered: Nov 2011
Posts: 447

 NOP               1A, 3A, 5A, 7A, DA, FA
 SKB               80, B2, C2, E2, 04,14, 34,44, 54, 64, 74, D4, F4
 SKW               0C, 1C, 3C, 5C, 7C, DC, FC

NOP No operation
SKB Skip byte (that is, branch of +1)
SKW Skip word of two bytes (that is, branch of +2)

2012-07-04 14:22

Skate

Registered: Jul 2003
Posts: 492

A small addition to Flavioweb's comment.

SKB/SKW are also called DOP/TOP (Double/Triple no OPerations) but since double and triple no operation equals to no operation, i'd like to call all of them NOP. But still, SKB and SKW are better descriptions since they really skip parameter parts.

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