[z80] initialization of globals

[sverx]

I see that globals explicitly initialized to some value get initialized thru a copy in crt0... but I don't get how/when other globals are initialized.
I mean, doesn't C require that every global is initialized to 0 when not explicitly set to a value?

[Philipp Klaus Krause]

Some platforms do the zeroing in the BIOS / loader, so it is left out of the default crt0.s (I guess for historic reasons, maybe the original developers of the z80 port were using such a platform). On platforms where this doesn ot happen, you need to zero the memory in your custom crt0.s (e.g. libcv, http://colecovision.eu/ColecoVision/development/libcv.shtml for the ColecoVision).

Philipp

[sverx]

So you mean that actually if I declare a global like this:

unsigned int myGlobal=0;

it will be initialized but if I declare it like this:

unsigned int myGlobal;

it won't get initialized so actually it could end up with a bogus initial value if I don't provide a system to clear RAM in crt0?

[Philipp Klaus Krause]

Yes. It doesn't make much sense, but that's how it currently is for historical reasons.

Philipp

[sverx]

OK. So, since you're not doing it in the provided z80 crt0 I guess you should write somewhere in the docs that globals won't be initialized if you don't do it explicitly.

Thanks for your help :)

[Maarten Brock]

Instead of documenting this I think we should change it first. The default should be to zero the memory for uninitialized globals/statics. And if you don't need that, you can remove it in your custom crt0.

[sverx]

I agree, but how can you do that? I mean I have this code added to my crt0:

xor a                ; clear RAM (to value 0x00)
ld hl,#0xc000        ; by setting value 0
ld (hl),a            ; to $c000 and
ld de,#0xc001        ; copying (LDIR) it to next byte
ld bc,#0x1ff0        ; for 8 KB minus 16 bytes
ldir                 ; do that

which of course works, but assumes RAM is at 0xC000 and 8 KB big, but that's of course platform dependent...

[Philipp Klaus Krause]

The linker provides symbols for location and length of of segments.

Philipp

P.S.: I'd write their names here, but Sourceforge markup keeps messing them up.