2010-02-27

Das Blinklicht with gcc-elf-m68k

The first program you write when you learn a language is the "hello world" classic, in a embedded system where you don't have a serial port (or you don't know if it is working) you have to use "das blinklicht" example. Er... it's just a blinking LED.
After setting up the cross compiler tool chain you'll need to set up the linker script and the crt0.s files.
The linker script defines where (in physical addresses) is the RAM and ROM of your system, which sections of code go into where and in my case the reset vector and initial stack pointer. There are still some things missing (namely libraries, floating point and multiplication/aritmetic), but I'm a step closer. I must admit that I found these (crt0.s and ldscript) on the net but I don't remember where or who did them. I've just change them a little.
Although the ROM is mapped on reset during the first 8 clocks at 0x00000, after the 8th clock ROM gets mapped at 0x80000. When you are programming a real eprom (or using my "linux-updated" rom emulator) you must move the code to the bottom of the ROM. This is accomplished with the command "AT".

GCC creates a section of code for "initialized data" called .data, this section is put into ROM and at the start it is copied into RAM (at CRT0.S), with the blinklicht program I don't use any .data or .bss or even stack, so for now I will not test if this works.

Without using optimizations -O2 your software loops will not be cleared by the optimizer, and it is pretty easy calculate the time taken for the software loops.

I had calculated an access to the MFP m68901 every 15us with a 4MHz clock and what I had was 30us.. I found the "problem" the next day, the instruction timing sheets are for the m68000 with its 16bit data bus, a 68008 takes two memory cycles for each of m68000, therefore twice as long.
The simple makefile is here

2 comments:

Günter Woigk said...

Thanks for this post.
This helped me starting with the software for my 68008 project. ;-)

Günter Woigk said...

Thanks for this post!
It helped me much with starting the software for my 68008 project! :-)