Differences

This shows you the differences between two versions of the page.

--- base:dysp_cycle_table [2016-04-26 14:35] – ; compyx
+++ base:dysp_cycle_table [2016-04-26 14:56] – compyx
@@ Line 47: / Line 47: @@
 </code>
+By altering the branch instruction to skip a variable amount of bytes of code, we can determine the number of cycles the loop must waste on a raster line. Generally speaking, the more sprites we have on a line, the more cycles the VIC eats, which means we have to branch further to skip more cycles.
+The "cpx #$e0 ... bit $ea" code is used to waste cycles with an accuracy of a single cycle, I use the same trick to time VSP's (with a few more "cpx #$e0" instructions).
+Here's how it works (simplified):
+<code 6502tasm>
+; here we waste 2 + 2 + 3 = 7 cycles:
+.1000   bpl $1002
+.1002   cpx #$e0      ; 2
+.1004   cpx #$e0      ; 2
+.1006   bit $ea       ; 3
+; adjusting the branch, we can waste 6 cycles:
+.1000   bpl $1003     ; we branch into the argument of CPX #$e0 at $1003
+.1002   cpx #$e0      ; which means we execute CPX #$e0 at $1003,
+.1004   cpx #$e0      ; then CPX #$24 at $1005
+.1006   bit $ea       ; and finally NOP at $1007
+; so, for the above code , the CPU executes this:
+.1000   bpl $1003
+; .1002 gets skipped
+.1003   cpx #$e0      ; 2
+.1005   cpx #$24      ; 2
+.1007   nop           ; 2
+</code>
+So for each additional byte we branch over, we either end at BIT $24 (3 cycles) or NOP (2 cycles).