Hallo Marko, > In case anything that is incompatible with the NMOS 6502 is considered an error, I could offer two candidates. > > (1) The BCD mode is not clearly covered. > ... > The instruction decoder ROM should be able to represent any illegal opcodes just fine. To start with the Instruction Decoder ROM: in this case I use EEPROMs. As input they use the Opcode, a counter and various state signals like Reset, NMI, IRQ and whether a branch should be taken or not. IMHO _every_ opcode can be programmed, step by step, thus also steps that actually are just a waist of time. And with "every opcode" I thus also mean illegal opcodes. But I haven't studied them very well and there may be some exceptions: illegal opcodes that make various registers output their data at the same time. IIRC SAX (Store A and X) does this. And that could ruin my 74ALS573 latches. Regarding the BCD: when I started with the design I first had the idea to use the well known 74181 ALUs. Until I ran into the BCD mode. And that is when I decided to use two EEPROMs as ALU as well. If you have a look at the schematics you will seen that one of the inputs is the decimal flag. And again it is just a matter of programming these EEPROMs to behave like the ALU inside the 6502. I just mentioned TWO EPROMs. And yes, this ALU is 16 bits, thus four EEPROMs. The original design had only two but then I needed something to handle indexed commands and the first design had extra adders to take care of this. But the I thought about a 16 bits ALU because: a) a 16 bits ALU could solve the needed additions as well b) I needed much less parts c) as bonus I could think of 16-bits operations (65816) I hope this clears the matters. Met vriendelijke groet / With kind regards, Ruud Baltissen www.Baltissen.org De informatie in dit e-mailbericht is vertrouwelijk en uitsluitend bestemd voor de geadresseerde. Wanneer u dit bericht per abuis ontvangt, verzoeken wij u contact op te nemen met de afzender per kerende e-mail. Verder verzoeken wij u in dat geval dit e-mailbericht te vernietigen en de inhoud ervan aan niemand openbaar te maken. Wij aanvaarden geen aansprakelijkheid voor onjuiste, onvolledige dan wel ontijdige overbrenging van de inhoud van een verzonden e-mailbericht, noch voor daarbij overgebrachte virussen. APG Groep N.V. is gevestigd te Heerlen en is ingeschreven in het handelsregister van de Kamer van Koophandel Limburg onder nummer 14099617 The information contained in this e-mail is confidential and may be privileged. It may be read, copied and used only by the intended recipient. If you have received it in error, please contact the sender immediately by return e-mail; please delete in this case the e-mail and do not disclose its contents to any person. We don't accept liability for any errors, omissions, delays of receipt or viruses in the contents of this message which arise as a result of e-mail transmission. APG Groep N.V. is registered in the trade register of the Chamber of Commerce Limburg, The Netherlands, registration number: 14099617 Message was sent through the cbm-hackers mailing listReceived on 2016-05-30 12:00:02
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