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View topic - strange code in Alex Kidd - The Lost Stars

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  • Joined: 03 Aug 2017
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strange code in Alex Kidd - The Lost Stars
Post Posted: Mon Aug 05, 2019 10:51 pm
Hi everyone,

I'm currently looking at some disassembly of "Alex Kidd - The Lost Stars". During the initialization some strange things happen:

The code I'd like to discuss starts at address 0352h. 3 register pairs get initialized:

_LABEL_352_23:
   ld   de, $0515
   ld   hl, $C000
   ld   bc, $0020

That seems absolutely normal so far :-). Then it goes on:

   ex   de, hl

Why exchange DE with HL and not initialize it correctly in the first place? My confusion goes on:

   ld   d, $C0

D is already C0h here...

   ld   a, e
   add  a, $1C

First, A is set 0 (because E is 0), just to add 1Ch afterwards. Why not just "ld a, $1C"? But as we see below, A get's overwritten anyway.

   ld   e, a      ; E = 1Ch
   ld   b, c      ; B = 20h
   ld   c, $80      ; C = 80h

Again some useless (?) juggling around...

_LABEL_365_24:
   ld   a, (hl)

Frome here everything seems quite reasonable, but everything above could have been replaced with:

   ld   de, $C01C
   ld   hl, $0515
   ld   bc, $2080

Can somebody explain this code to me? What I can imagine:

A) some code generation took place which produced a not very well optimized code
B) same as A, but with a compiler
C) I missed an important point here

I very much favour C) ;-).

Thanks in advance for your help!


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Post Posted: Mon Aug 05, 2019 11:39 pm
My guess is that the constants weren't always these values so it made more sense at the time, and the values got poked in after assembly.
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