how is load""code autostart implemented?

[777]

cant see anything on google about it. any help would be appreciated.

[TheMartian]

If I'm correct, I think that *in general* you can't.

Say, imagine you LOAD "" CODE a block at $8000, you need to call USR 32768.

However, if you're loading over the system variables area, you can overwrite the variables that contain the immediate command being executed.

Say you save with:

SAVE "block" CODE 23296,30000: RANDOMIZE USR 28000

When you LOAD "" CODE, you load the program, but also the system variables and the SAVE:RANDOMIZE commands. When the LOAD routine returns, it's like you just returned from the SAVE routine, and go into the RANDOMIZE USR.

Does this answer your question?

[777]

i think so. thank you

[Bedazzle]

If I'm correct, I think that *in general* you can't.

In general, you can.
Let's examine Eric and the floaters loader



TZX itself contains these files:



So, by CLEAR it is changing where "safe" area of memory starts. It means, Basic do not touch anything after that address.
Additionally, stack is moved there.
Code block loaded from address 32722 overlaps stack area, and after loading is finished, just jumps to address, that was loaded with code block.

Here goes hex dump of beginning of file:



You can see $9FD0 address, and if we examine what is there in memory...



Yeah, this is decryptor. When it finishes, execution jumps to $8000, or 32768 decimal, what is equal to address, where is original game entry point.

[TheMartian]

Oh yes I missed that case, sorry. Overwriting the stack so the return from LOAD "" CODE goes to the game.

[sn3j]

When you LOAD "" CODE, you load the program, but also the system variables and the SAVE:RANDOMIZE commands. When the LOAD routine returns, it's like you just returned from the SAVE routine, and go into the RANDOMIZE USR.

I remember this back then, when one of my tapes wore out I tried to load the code somewhere else in memory and save it back to a fresh tape. Then, after franctically fiddling with my 'new'-but-just-reused tape and the recorder's audio, it finally loaded but just refused to run. Quite mind-boggling for me as I had no idea of the concept.

[patters]

Interesting stuff. I remember now that I had one such tape and I used to wonder what kind of sorcery had been used

[Pobulous]

Easiest way is to do something like

10 SAVE "GAME" CODE 16384,49152
20 RANDOMIZE USR whatever

When you load the code back in it will resume from line 20.

[Ast A. Moore]

I explained it here.

[sn3j]

One of those games was Pogo.
I remember it because it was packaged with the ZX 48K here in Germany.
It featured a small Basic program that loaded a huge code chunk from 16384 to 65535.
This took around 5 minutes to load and you were praying to the recorder that it got the audio right.
If something was tripped up you mostly wouldn't notice until the last bit was read in and you were left with an error screen.
I had no idea how to hack this. And tape to tape copying used to detoriate the signal further.
The cheap cassette players that you could get hold of with your pocket money used to wear out tapes quite easily.

https://spectrumcomputing.co.uk/entry/3 ... ctrum/Pogo

[Dr beep]

One of those games was Pogo.
I remember it because it was packaged with the ZX 48K here in Germany.
It featured a small Basic program that loaded a huge code chunk from 16384 to 65535.
This took around 5 minutes to load and you were praying to the recorder that it got the audio right.
If something was tripped up you mostly wouldn't notice until the last bit was read in and you were left with an error screen.
I had no idea how to hack this. And tape to tape copying used to detoriate the signal further.
The cheap cassette players that you could get hold of with your pocket money used to wear out tapes quite easily.

https://spectrumcomputing.co.uk/entry/3 ... ctrum/Pogo

How to hack POGO, POLE POSITION and A VIEW TO A KILL

[Bedazzle]

How to hack POGO, POLE POSITION and A VIEW TO A KILL

Back in time real jedi did this from machine code

loading game file to address 25000
save "first" code from 25000 with length 40536
(screen can be saved separately to 6912, and crunched)

load game code again, but to an address 0 (ROM)
save "second" code from 40536 with length 8616

then these data blocks can be easily loaded and explored with any disassembler, like MONS or STS.

[Dr beep]

Back in time real jedi did this from machine code

loading game file to address 25000
save "first" code from 25000 with length 40536
(screen can be saved separately to 6912, and crunched)

load game code again, but to an address 0 (ROM)
save "second" code from 40536 with length 8616

then these data blocks can be easily loaded and explored with any disassembler, like MONS or STS.

I see a few flaws here
1) loading first blocks could cause basic to make code run which is not loaded (second block)
2) second block could overload stackpointer and make it run as well.
3) still knowledge needed to make it loadable

My program only needs pressing space and the use of a taperecorder.

[spider]

CODE 23552, length

Can calc length although these days I cheat and truncate it with ZXBE, with care.

Here:

Tape contains loader in Basic:

Code: Select all

10 LOAD "" CODE

And the 'Basic' in the code:

Code: Select all

10 REM
20 REM BORING TEST PROGRAM
30 REM
40 POKE (RND*767),RND*127
50 IF INKEY$<>"" THEN CLS
60 GOTO 40

Download here > https://ufile.io/ppzxz0en (should stay "alive" for about 30 days) long enough for what it is.

[Bedazzle]

I see a few flaws here

Seems, you missed the whole stuff about machine code, and disassemblers.

[Dr beep]

Seems, you missed the whole stuff about machine code, and disassemblers.

No I did not. You still need knowledge to add those together.
My "48 Kraak" does the whole job and copyable with a copyprogram that can handle larger blocks.

[chupocro]

There was another way of autostarting the CODE block which was not mentioned. While loading the data would overwrite ERRSP system variable and then an error was caused intentionally by the code stored on tape being one byte shorter that it should be so the parity byte was missing.