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<TITLE>Technical info</TITLE> |
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<H1>Technical info</H1> |
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"Known?" said Gandalf.<BR> |
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"I have known much that only the Wise know, Frodo." |
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</CITE> |
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Frodo tries to exactly imitate C64 hardware features. Now the 64's hardware |
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(esp. the graphics chip "VIC-II") has a rather simple design resulting in |
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many of the internal processes being visible to the "outside". So there are |
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lots of "undocumented features" you can do effects with the designers never |
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dared to dream about.<P> |
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Frodo uses a line-by-line emulation, i.e. the function of the VIC and the |
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processor (6510) are emulated for one raster line of the C64 screen at |
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times. In practice, Frodo runs VIC and 6510 alternately for 63 simulated |
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cycles each (corresponding to one raster line). At first, it emulates the |
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processor for 63 cycles, then switches over to the VIC that paints one |
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pixel row to the screen, then again 63 cycles processor, and so on... If |
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the 1541 processor emulation is turned on, 6510 and 6502 (in the 1541) |
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instructions are executed by Frodo in an interleaved fashion.<P> |
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Even though this is a heavy simplification of the processes in a real C64, |
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it lets you perfectly emulate many graphical effects possible on the C64, |
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e.g. FLD, DYCP, hyperscreen and many more. But this method has one big |
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disadvantage: Changes made to VIC registers by the processor in the middle |
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of a raster line will only take effect at the start of the next line. E.g. |
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you can't change the border color in the middle of a line, the color change |
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takes place in the next line. Therefore, very sophisticated techniques |
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depending on the exact position of a register change can't be emulated. For |
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instance, it is no problem to open the top and bottom border, but opening |
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the left and right border is impossible (and therefore not implemented in |
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the emulation).<P> |
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Frodo SC goes one step further by switching between VIC and 6510 in every |
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cycle and precisely emulating the internal functions. Modifications to |
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VIC registers become visible immediately in the next clock phase and |
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therefore it can even emulate effects that depend on the exact position |
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of a register change within a raster line, e.g. special FLI routines, |
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opening the left/right border, linecrunch, DMA delay, multiple repeated |
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sprite lines and executing programs in open address spaces ($de00-$dfff) |
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and in the color RAM. The 6510 emulation is also more precise and does |
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the same memory accesses as the real 6510, even the "unnecessary" ones |
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that come from design weaknesses of the 6510 and are not needed for the |
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function of single opcodes (e.g. in an instruction sequence like |
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INX:INX:INX:INX, the 6510 reads every opcode twice).<P> |
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A detailed technical description of the VIC-II can be found in an |
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<A HREF="http://www.cebix.net/VIC-Article.txt">article</A> I wrote. |
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