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by *lft*

Sometimes we want to delay a variable number of cycles, e.g. during VSP or when setting up a stable raster.

A typical way of handling a variable delay is to use a clock slide. In the following example, we start somewhere in a given range of cycles, we want to end up exactly at cycle 50, and we've computed the corresponding number of cycles to skip in A.

         ; number of cycles to skip ahead in A
         sta     branch+1    ; 31..41
branch   bpl     *           ; 35..45
         lda     #$a9        ; 38
         lda     #$a9        ; 40
         lda     #$a9        ; 42
         lda     #$a9        ; 44
         lda     $eaa5       ; 46
         ; at cycle 50

Notice that the operands are reinterpreted as opcodes depending on where we land on the slide. For instance, the final lda $eaa5 is encoded as ad a5 ea and takes 4 cycles. Skipping the first byte, we get a5 ea (lda $ea, 3 cycles). Skipping also the second byte, we get ea (nop, 2 cycles).

The length of the clockslide depends on the maximum jitter we have to support, and we pay a corresponding penalty in the form of useless waiting cycles. In the example, the maximum supported jitter is 10 cycles (41-31), and the minimum overhead cost is 9 cycles (50-41).

Now here comes the improvement: