A jump table
Four addresses in memory, an index that picks one of them, and a jmp through an address register instead of a chain of comparisons.
A number picks which of four pieces of code runs. d2 holds 2, the program reads the third address
out of a table in memory and jumps to it, and the multiplication is what happens. Changing d2
changes the answer without changing a comparison anywhere.
The bigger of two numbers chose between two paths with a cmp and a branch. A chain of those works
for three or four cases and gets slower with every one you add, since a value at the bottom of the
chain is compared against everything above it first. A table is looked up once whatever the value is.
You need to know: the "Compare and branch" lecture and the "Addressing modes" lecture. What is
new here is a jump to an address the program worked out, jmp (a1) goes to whatever a1 holds,
which nothing in the source names.
dc.l add_op, sub_op, mul_op, div_op writes four longs, and each one is the address the assembler
gave that label. Put the memory panel on 2000 and they read 00001020, 0000102C, 00001038 and
00001044, which are the four addresses inside your own code. A label is nothing but an address, and
this is what that sentence is for.
The three instructions before the jmp are C's table[op]: multiply the index by the size of an
element with a shift, add it to the base with the indexed mode, and read the long there. What comes
out is an address, so it goes into an address register. jmp (a1) then leaves without
pushing anything, which is what makes this a switch and not four calls; jsr (a1) would make it
four calls.
d4 comes out at 00000012, which is 18, and a1 at 00001038, the address of mul_op. Each arm
ends with bra done for the same reason the two halves of an if do: the arms are laid out one
after another and nothing stops the program running into the next one.
Try changing move.l #2, d2 to move.l #3, d2 and d4 comes out at 2, which is 6 divided by 3.
With #0 it comes out at 9. Nothing else in the program moves, and there is no comparison anywhere
in it to change.