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The Gigatron project
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H.G.Muller



Joined: 10 Mar 2006
Posts: 21547
Location: Amsterdam

PostPost subject: Re: The Gigatron project    Posted: Wed Dec 06, 2017 12:15 pm Reply to topic Reply with quote

Indeed, that is about it. But that doesn't make it very different from the celebrated 6502. (The lack of condition is a bit of a downer, though.) And because of the Harvard architecture it tends to outperform the latter. Even though there is no 16-bit addressing mode, you can load the high address in Y, and use it as a page register. Then the access takes two clock cycles, but on 6502 absolute addressing took 4 clocks cycles: fetch of opcode and both address bytes, and then the access. Even when Y needs to be preserved you can afford to save a copy of it in zero-page memory, and load Y from it again after use as page register, ad you are still not off any worse. But in many cases, Y doesn't need to be preserved, so you are 2 cycles faster. And in other cases, Y has to be restored only after a bunch of uses as page register, or to the same value after a number of such bunches (so you can use the same stored value).

I have to get used a bit to the absence of a real indexed mode, however. It took me some time to discover that the best way to allocate global tables is 'vertically', i.e. not as contiguous memory, but one byte in each memory page (at the same offset), so that it can be indexed by the high byte of the address, through the Y register.
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Subject Author Date/Time
The Gigatron project H.G.Muller Tue Dec 05, 2017 6:32 pm
      Re: The Gigatron project H.G.Muller Tue Dec 05, 2017 7:12 pm
            Re: The Gigatron project H.G.Muller Tue Dec 05, 2017 9:45 pm
                  Re: The Gigatron project H.G.Muller Wed Dec 06, 2017 2:44 pm
                        Re: The Gigatron project H.G.Muller Wed Dec 06, 2017 11:05 pm
                              Re: The Gigatron project Rémi Coulom Wed Dec 06, 2017 11:51 pm
                              Re: The Gigatron project H.G.Muller Sun Dec 10, 2017 11:51 am
                                    Re: The Gigatron project H.G.Muller Sun Dec 10, 2017 10:10 pm
                                          Re: The Gigatron project Rasmus Althoff Sun Dec 10, 2017 10:20 pm
                                                Re: The Gigatron project H.G.Muller Sun Dec 10, 2017 10:30 pm
                                          Re: The Gigatron project Stefano Gemma Mon Dec 11, 2017 11:15 am
                                                Re: The Gigatron project H.G.Muller Mon Dec 11, 2017 12:15 pm
                        Re: The Gigatron project Rasmus Althoff Thu Dec 07, 2017 9:44 pm
      Re: The Gigatron project Stan Arts Tue Dec 05, 2017 7:23 pm
      Re: The Gigatron project Dann Corbit Tue Dec 05, 2017 8:11 pm
            Re: The Gigatron project H.G.Muller Tue Dec 05, 2017 8:58 pm
      Re: The Gigatron project Rasmus Althoff Tue Dec 05, 2017 9:04 pm
      Re: The Gigatron project Fulvio Benini Wed Dec 06, 2017 8:31 am
      Re: The Gigatron project Martin Sedlak Wed Dec 06, 2017 11:42 am
            Re: The Gigatron project H.G.Muller Wed Dec 06, 2017 12:15 pm
                  Re: The Gigatron project Martin Sedlak Thu Dec 07, 2017 10:48 pm
      Re: The Gigatron project Ian Osgood Thu Dec 07, 2017 12:17 am
      Re: The Gigatron project Rémi Coulom Thu Dec 07, 2017 9:58 pm
            Re: The Gigatron project H.G.Muller Fri Dec 08, 2017 8:45 am
                  Re: The Gigatron project Stefano Gemma Fri Dec 08, 2017 11:07 pm
                        Re: The Gigatron project H.G.Muller Sat Dec 09, 2017 1:16 pm
                              Re: The Gigatron project Stefano Gemma Sat Dec 09, 2017 1:44 pm
                                    Re: The Gigatron project H.G.Muller Sat Dec 09, 2017 2:21 pm
                  Re: The Gigatron project Rasmus Althoff Sat Dec 09, 2017 7:27 pm
                        Re: The Gigatron project H.G.Muller Sat Dec 09, 2017 8:29 pm
                        Re: The Gigatron project Charles Roberson Sat Dec 09, 2017 9:30 pm
                              Re: The Gigatron project Rasmus Althoff Sat Dec 09, 2017 9:36 pm
                                    Re: The Gigatron project Martin Sedlak Sat Dec 09, 2017 9:57 pm
                                          Re: The Gigatron project Rasmus Althoff Sat Dec 09, 2017 10:10 pm
                                                Re: The Gigatron project H.G.Muller Sat Dec 09, 2017 11:31 pm
                                                      Re: The Gigatron project Stefano Gemma Sun Dec 10, 2017 9:01 am
                                                            Re: The Gigatron project H.G.Muller Sun Dec 10, 2017 11:32 am
                                                                  Re: The Gigatron project Stefano Gemma Sun Dec 10, 2017 12:24 pm
                                                                        Re: The Gigatron project H.G.Muller Sun Dec 10, 2017 8:44 pm
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