If I correctly understand your application, Erik's [3] doesn't sound that dramatically bad. You'd use MOVC for reading, won't you - and that's quite fast, isn't it. You can limit the number of needed reads by performing the "garbage collection" either when for a certain variable Nth write is reached, or periodically during "idle" times, if such occur in the application.

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There's no trick in B52, the clumsiest possible method is used, employing a sector-sized buffer in RAM, and no provision for sudden powerdown. The BASIC program is stored in FLASH in the same format as in RAM, "linearly", so if lines are added at the end, only those are written into the FLASH, but if a line is modified or inserted or removed from somewhere in the middle, a significant FLASH rewrite is performed - it is quite perceptible as soon as the program reaches a couple dozen of lines. But then I don't think this particular application needs more sophistication than that.

Jan