Re: Porting GCC to 8051 Microcontroller

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Porting GCC to 8051 Microcontroller
\ Ganesh (7 May 2008)
. \ Fabian Cenedese (7 May 2008)
. . \ Ganesh (7 May 2008)
. \ Andrew Haley (7 May 2008)
. \ Ganesh (7 May 2008)
. . \ Andrew Haley (7 May 2008)
. . . \ Andrew Haley (9 May 2008)
. \ Duane Ellis (9 May 2008)

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Subject:	Re: Porting GCC to 8051 Microcontroller
Group:	Gcc-help
From:	Duane Ellis
Date:	9 May 2008

andrew>> I've been thinking about this some more. I
andrew>> suspect you could get good results by
andrew>> generating not 8051 assembly language,
andrew>> which would be gross, but a simple bytecode engine.

I know this for a fact :-) in a previous life we modeled
the one we did off the instruction set like the MIPS.
Why? Simple - it has to do with the way the flags
register is done.

On some CPUs - when you add "r0 + r1" => r2, you either
*must* update the flags (zero, carry, overflow) or not.
having to maintain that in a simulator is added wasteful
steps that you often do not need...

For example the test:

if( a == b ){
do_this();
}

For us the opcodes became:
test r1=r2 ;; test(r1==r2)
braf +4 ; branch if false
jsr24 do_this ; 24bit relative call.

This simplified the interpretation code and was a
huge performance win on a 6502 @ 1mhz.

The simulation engine was a big 6502 switch statement.

In effect, the 6502 was a micro-code engine :-)

-Duane.