Technical info

"Known?" said Gandalf.
"I have known much that only the Wise know, Frodo."

Frodo tries to exactly imitate C64 hardware features. Now the 64's hardware (esp. the graphics chip "VIC-II") has a rather simple design resulting in many of the internal processes coming to the "outside". So there are lots of "undocumented features" you can do effects with the designers never dared to dream about.

Frodo uses a line-by-line emulation, i.e. the function of the VIC and the processor (6510) are emulated for one raster line of the C64 screen at times. In practice, Frodo runs VIC and 6510 alternately for 63 simulated cycles each (corresponding to one raster line). At first, it emulates the processor for 63 cycles, then switches over to the VIC that paints one pixel row to the screen, then again 63 cycles processor, and so on... If the 1541 processor emulation is turned on, 6510 and 6502 (in the 1541) instructions are executed by Frodo in an interleaved fashion.

Even though this is a heavy simplification of the processes in a real C64, it lets you perfectly emulate many graphical effects possible on the C64, e.g. FLD, DYCP, hyperscreen and many more. But this method has one big disadvantage: Changes made to VIC registers by the processor in the middle of a raster line will only take effect at the start of the next line. E.g. you can't change the border color in the middle of a line, the color change takes place in the next line. Therefore, very sophisticated techniques depending on the exact position of a register change can't be emulated. For instance, it is no problem to open the top and bottom border, but opening the left and right border is impossible (and therefore not implemented in the emulation).

Frodo SC goes one step further by switching between VIC and 6510 in every cycle and precisely emulating the internal functions. Modifications to VIC registers become visible immediately in the next clock phase and therefore it can even emulate effects that depend on the exact position of a register change within a raster line, e.g. special FLI routines, opening the left/right border, linecrunch, DMA delay, multiple repeated sprite lines and executing programs in open address spaces ($de00-$dfff) and in the color RAM. The 6510 emulation is also more precise and does the same memory accesses as the real 6510, even the "unnecessary" ones that come from design weaknesses of the 6510 and are not needed for the function of single opcodes (e.g. in an instruction sequence like INX:INX:INX:INX, the 6510 reads every opcode twice).

A detailed technical description of the VIC-II can be found in an article I wrote (32k gzipped).