Answer Posted / mikew

The switch branches on one value only, whereas the if-else
tests multiple logical expressions.

So you could say that the switch is a subset of if-else.

The potential difference if that switch is conceptually an
N-way branch point, whereas the if-else is always a
(repeated) binary branch.

However, if you are checking, say, a return code, against a
varied list of possibilities, then the switch can give
greater clarity to source code - and allow simpler addition
of new cases, making it easier for maintenance, as well as
allowing the compiler to generate simpler code.

Simpler code is possible because it can generate a jump
table to perform the multiple comparisons, i.e. in pseudo-code:

_jump_table:
DEFW case1_address
DEFW case1_value
...
DEFW caseN_address
DEFW caseN_value
DEFW NULL /* terminator */

- search _jump_table for case_value
- branch to corresponding case_address, or take default
action if NULL terminator found instead.

In the special case that case1 ... caseN values are
sequential numbers (maybe with a few gaps) then the table
and code can be further simplified as a simple indexed branch:

_jump_table:
DEFW case1_address
...
DEFW default_address /* fill in any holes !*/
...
DEFW caseN_address

- check value is between case1_value and caseN_value
- subtract case1_value
- load branch address word from _jump_table word-indexed on
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