Il 14/08/2018 15:21, Marco Borsari via fpc-pascal ha scritto:
Why the code below does exit gracefully without prints anything?
Sure, it is for my poor knowledge of the assembler, but in some
details, please...
program branch;
{$ASMMODE intel}
label next,stop,a,b,c;
var idx:byte;
begin
write('Index? ');
readln(idx);
asm
mov ax,idx;
shl ax,2;
mov bx,next;
add bx,ax;
jmp (*short*) [ebx+4];
next:
jmp a;
jmp b;
jmp c;
end['EAX','EBX'];
a:
writeln('0');
goto stop;
b:
writeln('1');
goto stop;
c:
writeln('2');
stop:
writeln('stop');
end.
Just another question: why the short modifier is unrecognized?
Thanks for any help in this holydays time,
Marco
I tested your code in my environment (Linux - x86_64), compiled with fpc
-g -a branch.pas. (-g is to add debug information, -a to get the
assembler listing).
Then launched it with gdb debugger:
>gdb branch
(gdb)run
The output is the following:
(gdb) run
Starting program: /home/colla/Applicazioni/Lazarus/Branch/branch
Index? 1
Program received signal SIGSEGV, Segmentation fault.
main () at branch.pas:13
13 jmp (*short*) [ebx+4];
The first thing I notice is that you load BX, which is a 16 bit register
(the lower 16 bits of EBX), with the value of "next", and then Jump to
the content of EBX which is a 32 bit register, whose lower 16 bits have
been loaded but whose upper 16 bits are undefined. A SIGSEV is therefore
to be expected. If you are in a 32 Bit environment you should use EBX,
if you're in a 64 Bit environment you should use RBX.
What I would do, if I were in your place, would be to rewrite your
program in pure Pascal, compile it with the -a switch to get the
assembler listing, and use the compiler generated assembler code as a
guideline to your optimized assembler.
Giuliano
--
Do not do to others as you would have them do to you.They might have different
tastes.
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