Hi,
Brandon Fosdick schrieb:
> sdcc is generating an awful lot of instructions just to set some pins.
Strictly speaking Keil uses 4 instructions
whereas SDCC uses only 3 instructions for:
P0DIR = 0x0;
while(1)
P0 = 0xFF;
Both compilers use 8 bytes of code memory and the
same number of cycles on a plain 8051 though.
The difference is in the startup code that is being linked.
Keil uses a clever scheme there.
SDCC links code that is not customized for your application
and uses generic code which f.e. includes code for an
external startup routine.
But you have (manual) control over this code (see Manual 3.12.1)
and if you f.e. link an adapted crtstart.asm
(empty section GSINIT2 so only setting the stackpinter is set)
then you'll have shaved off 14 bytes:)
sdcc --no-xinit-opt --main-return --iram-size 0x80 crtstart.rel t.c
you arrive at:
:020000040000FA
:1A000000020006020012758107E4787FF6D8FD0200037593007580FF80FBAB
:00000001FF
which (passed through d52 disassembler with "d52 -pngd t.ihx") is:
ljmp X0006 ; 0000 02 00 06 ...
;
X0003: ljmp X0012 ; 0003 02 00 12 ...
;
X0006: mov sp,#0x7 ; 0006 75 81 07 u..
clr a ; 0009 e4 d
mov r0,#0x7f ; 000a 78 7f x.
X000c: mov @r0,a ; 000c f6 v
djnz r0,X000c ; 000d d8 fd X}
ljmp X0003 ; 000f 02 00 03 ...
;
X0012: mov 93h,#0x0 ; 0012 75 93 00 u..
X0015: mov p0,#0xff ; 0015 75 80 ff u..
sjmp X0015 ; 0018 80 fb .{
You could strip that down yet another 4-5 bytes using ajmp
and removing one ljmp.
Considering it's about 5 years ago from Erik Petrich's commit
2004-03-17 for the initialization code it's strange that
noone yet (seems to have) posted a minimized crtstart.asm :^)
> Keil is putting the bulk of the code at 0x800 with a jmp at 0x0, but I
> don't see any reason it has to be that way.
It's because Keil's evaluation version is code size
limited to 2k(**) (for other limitations see Keil's website).
For target devices with 2k code memory (0x800 bytes)
this limitation would not have an effect so Keil locates
the bulk of the code to above 0x800.
> Nor do I see a way to get
> sdcc to do the same thing.
See option --code-loc if you need to do the same thing
with SDCC.
Also use D51 (Manual 6.4) to disassemble both Intel Hex Files.
You'll note a difference that can be changed with option --iram-size.
And a different SFR address is used for port initialisation.
Greetings,
Frieder
(**) Keil evaluation versions which allow for 4k code are
shipped with some low cost 8051 development kits.
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