Hello Sdcc users,
I've been using sdcc for a year or two, thanks to all the developers for the
sdcc project.
I'm writing to ask for help understanding how to manage ram usage with the
mcu8051. I've read over the sdcc manual several times and have searched
through the mailing list history. I'm hoping that someone can point me in the
right direction.
The issue is that with a __bit variable defined, and several dozen __data
variables defined, there is a large gap of unused ram between the register bank
and the bit variable.
I'm sure I could force usage of the unused ram locations by using absolute
'__at' addresses, but perhaps there's a way for sdcc to manage the ram layout
automatically.
Here is some sample code. This sample includes a function to demonstrate
overlay variables, which is shown in the memory map. Take out the function and
you'll get the same results. I included the function to show that overlay
variables work nicely.
// start of code sample
// SDCC : mcs51/gbz80/z80/z180/r2k/r3ka/ds390
// /pic16/pic14/TININative/ds400/hc08/s08
// 3.2.0 #8008 (Jul 6 2012) (MINGW32)
#include <8052.h>
char oneChar1[24];
char oneChar2;
__bit oneBit = 0;
void delay(int jj, int kk)
{
int j;
int k;
for(k=0; k < kk; k++) {
for(j=0; j < jj; j++) {
}
}
}
void main(void)
{
while(1) {
delay(100,200);
}
}
// end of code sample
Here is a portion of the memory map, showing the large gap of unused data ram:
Internal RAM layout:
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x00:|0|0|0|0|0|0|0|0|Q|Q| | | | | | |
0x10:| | | | | | | | | | | | | | | | |
0x20:|B|a|a|a|a|a|a|a|a|a|a|a|a|a|a|a|
0x30:|a|a|a|a|a|a|a|a|a|a|S|S|S|S|S|S|
Comment out the __bit variable and this is the memory map that results. You
can see the __data variables start right after register bank 0, followed by the
QQ overlay variables.
Internal RAM layout:
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x00:|0|0|0|0|0|0|0|0|a|a|a|a|a|a|a|a|
0x10:|a|a|a|a|a|a|a|a|a|a|a|a|a|a|a|a|
0x20:|a|Q|Q|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x30:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
Restore the __ bit variable and comment out the 'char oneChar2' variable and
you get this memory map, showing the __bit variable between the __data
variables and the overlay variables.
Internal RAM layout:
0 1 2 3 4 5 6 7 8 9 A B C D E F
0x00:|0|0|0|0|0|0|0|0|a|a|a|a|a|a|a|a|
0x10:|a|a|a|a|a|a|a|a|a|a|a|a|a|a|a|a|
0x20:|B|Q|Q|S|S|S|S|S|S|S|S|S|S|S|S|S|
0x30:|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|S|
Hope all this makes sense.
Thanks for your time,
Rich.
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