My Amiga clears memory always

[LeCaravage]

Hi,

Context : Amiga 500 KS1.3 OCS, 1Mb Chip Ram (Boobip ram trap door extension) + 1,5 F.Fast. DF0 real drive, DF1 : Hxc Drive. Oktagon HD 900Mb.

I tried to see some code of another program by reseting Amiga and right after loading Asmone (by inserting another disquette) but I didn't find anything, memory was cleaned.
And to be sure of it, I loaded severals other program and checked memory using "3rd Day" utility (gfx ripper), but same conclusion, memory seems to be cleaned during reset.
I'm puzzled why memory is always cleaned ?
I'm pretty sure it shouldn't with all programs. I plugged off my HD but no chance but I let the DF1 plugged in though.

Any clue what's going on ? Thanks

[ross]

It a 'bug' in KS1.3 and 1MB chip machines (it's the same problem that affects reset-proof programs in this very configuration).

If I'm not wrong SetPatch takes care of it.

[LeCaravage]

Ok, my Amiga is not bugged then
Thanks Ross.

[Karlos]

There's a irony for you. What is seen as a bug here is today a minimum security requirement for any remotely serious OS. The last thing you want us to be able to extract data from the memory of a machine after a reboot.

[a/b]

That typically happens when you trash critical part of exec's structures (it has a checksum for validation, and if it's incorrect it will do a "full init" including memory clear, at least that's what KS1.3 does).
Back in the early days my friends and I used to do that to protect our intros (not that they were anything worth protecting :P ). For example, this little instruction placed strategically will do that (a6=exec base):

  addq.w #1,36(a6)

[paraj]

Originally Posted by a/b

That typically happens when you trash critical part of exec's structures (it has a checksum for validation, and if it's incorrect it will do a "full init" including memory clear, at least that's what KS1.3 does).
Back in the early days my friends and I used to do that to protect our intros (not that they were anything worth protecting :P ). For example, this little instruction placed strategically will do that (a6=exec base):

  addq.w #1,36(a6)

Pretty advanced anti-debugging technique


The bug ross mentioned is part of the execbase validation logic. It considers less than 256K or more than 512K chipmem to be invalid.

FC0196  move.l    $3E(A6),A3        Get end address of chip memory.
FC019A  cmp.l     #$080000,A3       Greater than 512K?
FC01A0  bhi.s     FC01CE            If so, it must be invalid.
FC01A2  cmp.l     #$040000,A3       Less than 256K?
FC01A8  bcs.s     FC01CE            If so, it must be invalid.

From the KS1.2 exec disassembly but the exact same code is present in 1.3 (though it was fixed for the one included with the A3000).

[LeCaravage]

I read your answers and I feel like I know very little (or almost nothing) to system coding.
Is there a book to learn Amiga 500 system programming (asm preferably) for dummy you could advice ?