#17 Level 2 bootfiles too small crash the system

[Boisy Pitre]

Under Level 2: If a bootfile is too small (<24K) it will crash the CoCo 3. I have traced this to the kernel in two areas: F$Move in fmove.asm and the the code that copies user/system stacks in krn.asm. Specifically, the use of the MMU at $FFA5/A6 causes problems if the bootfile is too small, which causes certain data structures to be allocated "too high" in system RAM.

The F$Move problem seems relegated to 6809 only, and I have modified the 6809 code so that it does not use the stack (the stack was disappearing when the MMU switched out the block at $FFA5).

The problem code in krn.asm spans both 6809 and 6309. I'm not sure how to deal with it.

[Robert Gault]

I'm not sure if this is relevant to the bug but seems a curious treatment of the stack in krn.asm.

In the Level2 krn.asm, part of the initialization has
std <D.Proc set user process descriptor to $0600
adda #$02 set stack pointer to $0800
tfr d,s
inca set system stack to $0900
std <D.SysStk
The stack pointer with a 6809/6309 is set to the top of the stack and builds down. So why is regS set to $800 and D.SysStk set to $900? Shouldn't the reverse be true?

[Boisy Pitre]

Robert, the D.SysStk area appears to be used differently. I did a grep and found that it is loaded into S in fnproc.asm and fexit.asm. More study is needed.

[Boisy Pitre]

Some additional notes: it appears that the minimum safe bootfile size is $4D00 (19712) bytes. This comes from the fact that the boot track is located at $ED00 and KrnP2 should be at $A000 and no higher ($ED00-$A000 = $4D00). If KrnP2 is at $A000 then it is guaranteed that all kernel data structures will be allocated BELOW $A000, so the kernel's stack copy routine can safely use the $FFA5 ($A000-$BFFF) region as a temporary map in/map out place.