From: "Cyrano Jones" <cyranojones_lalp@...>
Dec 14, 2007
; SDCC crt0.s for a mailstation app
.module crt0
.globl _eventhandler=
.globl _graphics
.globl _strings
.globl _dunno
.globl _install
=
.area _CODE
jp _eventhandler
.dw _graphics
.dw _strings
.d=
w _dunno
jp _install
.area _HOME
.area _GSINIT
_gsinit::
.area _=
GSFINAL
ret
.area _DATA
.area _BSS
.area _HEAP
; Rather than d=
isturb the simplicity of the above code,
; I will put the comments down he=
re.
;
; ".module crt0" simply defines a name for the linker to use
; if it=
needs to report where an undefined symbol was used.
;
; ".globl somesymbol=
" tells the assembler that somesymbol
; will not be resolved until link ti=
me.
;
; ".area SOMEAREA" lets the linker group similar types of
; code or =
data from the various modules, into single
; contiguous areas in memory. =
;
; This first code area will be the entrypoint to our code.
; It is man=
datory that crt0.o be the first module linked
; for this reason.
;
; Here=
we just make a jump to "eventhandler", and
; leave a couple of pointers to=
resource data. The
; mailstation firmware expects these to be at the hea=
d of
; any app. I added a jump to an install routine, just so
; it too c=
an have a fixed address.
;
; We mention all the areas that our program will=
use to
; force the linker to place them in a particular order.
; The _CO=
DE & _DATA segments will be pegged to particular
; addresses by the link c=
ommand. _HOME, _GSINIT, & _GSFINAL
; will be in ROM, after the code segmen=
t. _BSS & _HEAP
; will be in RAM, after the data segment
;
; The compiler=
will place any code for initializing globals
; into the _GSINIT section. =
Here, we place a label as the
; first thing in _GSINIT, and a "ret" as th=
e last thing,
; allowing the globals defined in all modules to be run
; w=
ith a single call to _gsinit.
;
; Just what is crt0?
;
; crt0.o is a module=
that is automatically linked by SDCC
; when you link your code. I imagin=
e historically it
; contained code to run a crt terminal. Now SDCC uses i=
t
; to link any sort of startup code you may need for your
; program. Ty=
pically this would be:
;
; 1) init stack pointer.
; 2) set up interrupt=
s.
; 3) init the globals.
; 4) call your "main" function.
; 5) imple=
ment any exit code needed.
;
; We don't do any of that! :-)
;
; Since we a=
re going to run under the mailstations
; existing OS, we can skip 1 & 2, '=
coz they are done
; for us by ms firmware.
;
; We skip number 3 because =
our code is just an event
; handler, and we do not want our globals re-ini=
t'ed on
; every event. We will let our code call gsinit itself,
; when i=
t receives a "sig_init" from the mailstation's OS.
;
; As for 4 & 5, we don=
't need any exit code, we just
; return to the ms OS after each event we p=
rocess.
; Since we don't need to come back here, rather than
; call the =
"main" func, we just jump to it. This has
; the added advantage of leavin=
g the stack-frame as is,
; so we can access the passed event directly.
;
;=
Voil=E0!