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62 lines
2.8 KiB
HTML
62 lines
2.8 KiB
HTML
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<HTML>
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<HEAD>
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<TITLE>Technical info</TITLE>
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</HEAD>
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<BODY>
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<H1>Technical info</H1>
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<CITE>
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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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<HR>
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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 coming to the "outside". So there are lots
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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.uni-mainz.de/~bauec002/VIC-Article.gz">article</A>
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I wrote (32k gzipped).
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</BODY>
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</HTML>
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