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## Arbitrary Code Execution Discussion

### What i should note before learning ACE in german version? - Page 1

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#### What i should note before learning ACE in german version?

Posted by: Scroogeduck
Date: 2019-09-26 09:50:18
Hello guys
First a big thank to glitchcity for the good page and all the helpful users.
I used this page a lot of times (and years) and it was all the time very helpful :).
Back in days I had never problems to follow and use a glitch since I saw the ACE Glitch.
I know there are some item lists for the german version, but this is the first time i really would like to learn why something is happening and how to make an own setup.
I have read a lot of pages here and have some question before I try to get deeper in this glitch (atm i have read so much things i dont even know how to start right XD). Today or tomorrow i will read again the topic" Where to start get started…" and from then step to step…
1. First and most important thing for me is, I would like to use the glitch in the german yellow version with w sm.
So I have seen that this link was posted a lot of times: https://glitchcity.info/biglist.htm
Has the german version the same list (sorry if this is a stupid question)?
2. When I check the list I cant find a difference between Red/Blue/Yellow.
So why we dont use the same item list for a glitch when using wsm in yellow (I mean the same item list like for red/blue with 8F?).
3. Sherkel answered in this topic (https://forums.glitchcity.info/index.php?topic=7995.0)
"$D158 in English Red corresponds to $D15D in European Red, and so forth." -> so from english red to german red would be +5 is that right?
4. I read often about BGB Debugger. Does this make the thing easier or can we find the whole code of the game on a webpage?
5. Is there maybe a big thing i should know before starting for the german version? (I have read for the code +4 from english red/blue and +5 from english yellow).
Thanks very much for some answers and have a nice evening :)
Best regards
Scroogeduck

#### Re: What i should note before learning ACE in german version?

Posted by: Sherkel
Date: 2019-09-26 14:03:10
Welcome to GCL! :) Good to hear it's been so helpful, and I hope it will continue to be.

> 1. First and most important thing for me is, I would like to use the glitch in the german yellow version with w sm.
> So I have seen that this link was posted a lot of times: https://glitchcity.info/biglist.htm
> Has the german version the same list (sorry if this is a stupid question)?

Yes. This is what you're looking for: https://glitchcity.info/wiki/ItemDex/Y:099#In_other_European_versions
(The excessive use of # signs there makes it pretty annoying to search, actually, in addition to not looking good.)

> 2. When I check the list I cant find a difference between Red/Blue/Yellow.
> So why we dont use the same item list for a glitch when using wsm in yellow (I mean the same item list like for red/blue with 8F?).

If I understand what you're asking correctly, it's that the garbage code that Red/Blue 8F executes isn't the same as the portion Yellow 8F does. None of it was intended to be read as code in the first place, so you can have major differences between versions there even if a Potion heals 20 HP in both.

> 3. Sherkel answered in this topic (https://forums.glitchcity.info/index.php?topic=7995.0)
> "$D158 in English Red corresponds to $D15D in European Red, and so forth." -> so from english red to german red would be +5 is that right?

Yep!

> 4. I read often about BGB Debugger. Does this make the thing easier

Oh, definitely. From a beginning level, it's perfect for testing item setups.

> or can we find the whole code of the game on a webpage?

Thanks to a handful of unbelievably dedicated souls, yes!! https://github.com/pret/pokeyellow

> 5. Is there maybe a big thing i should know before starting for the german version? (I have read for the code +4 from english red/blue and +5 from english yellow).

Maybe the French site PRAMA Initiative, as it deals particularly with another European version. I don't think it's strictly necessary so long as you can read the material here, though.
(Nitpick: I wouldn't call ACE a glitch, just something that glitches make possible.)
Good luck!

#### Re: What i should note before learning ACE in german version?

Posted by: Torchickens
Date: 2019-09-27 07:06:42

> 2. When I check the list I cant find a difference between Red/Blue/Yellow.
> So why we dont use the same item list for a glitch when using wsm in yellow (I mean the same item list like for red/blue with 8F?).

This can be because many codes write or read from a RAM address with the address defined by the items/quantity, in which case the +5 difference applies.
e.g.
Lemonade x 21
TM34 x 88
TM08 x 201
Here (classic wild encounter code) you're writing 0x15 to D058, where the hex:58 is represented by dec:88. So porting it to non-English EU Yellow you'd need to change it to TM34 x 93.
In another case, where the "call" or "jp" instructions to ROM are involved, the locations of functions in the ROM can differ between both languages and versions.
This is also a good disassembly resource like Sherkel posted (the Pokémon Red one is the original and is really thorough).
https://github.com/pret/pokered
Hope this helps! :)

#### Re: What i should note before learning ACE in german version?

Posted by: Scroogeduck
Date: 2019-09-27 12:46:38
Hey guys
Thanks very much for the answers and the links :).
I will check all the things the next days/weeks and i hope to understand it more or less :).
Have a nice weekend
Best regards
Scroogeduck

#### Re: What i should note before learning ACE in german version?

Posted by: metalmario32
Date: 2019-10-03 06:13:32
It's rare to see people using EU non-English (except French) games, and welcome to GCL!
So yes, a lot of what they've already covered above is true, you need to account for differences in addresses, for example, most addresses in the DXXX range and some starting with CFXX have the offset of +5 from the English Yellow. Also, the ACE setups and codes will be different. There are also a host of exclusive glitches on the German version, including more selectable characters for the old man trick, which can lead to some interesting results. (Of course, old man trick only works on German Red/Blue, however.)
Anyways, thanks for stopping by, and have a nice day!

#### Re: What i should note before learning ACE in german version?

Posted by: Parzival
Date: 2019-10-03 11:57:33

> EU non-English (except French) games

Run.
Before ISSOtm finds you.

#### Re: What i should note before learning ACE in german version?

Posted by: Scroogeduck
Date: 2019-10-10 10:15:07
I have read now a lot of pages and all was a little bit a mess. So i started to read every single post here in the ACE Section starting by last post.
So i came to six new questions^^.
1. Question:
ISSOtm posted this page: http://glitchcity.info/wiki/index.php/GB_Programming#A_new_world
So now im reading it and it makes a lot, really a lot clearer for me, its like i can see a little light at the end of the tunnel :).
I have a question there about a calculation in "Register pair and RAM" section. This is a calculation result from author:
sub a, b ; A = A - B = $06 - $DE = $06 + (-$DE) = $06 + ($21 + $01) = $28, C flag = 0
I calculated it i think in a really "noob" Version. I calculated $06 = 6 / $DE = 222
So 6 - 222 (and 0 is 256 / so 256 - 216) i come to 40 / $28 - okey its right.
But i would like to understand how the writer split -$DE into $21 and $01. I tried to understand and searched above in his post, but i really dont get it.
2.Question:
I have read and checked a video on youtube that a hex editor and a RAM Searcher is very good to use.
Im not very familiar with BGB Debugger atm. Is there such a thing implemented, like a all in one solution? Or do i need RAM Searcher and hex editor seperately?
3. to 5. Question:
Evie the Mother Hen has written in "Where to get started" this code:
ld a, 15
ld [$d059] ,a
ret
so atm im trying to find out by myself first which item and quantity i need and then i check with this page: https://eldred.fr/gbz80toitems3/
So im checking in big hex list the function for ld a,x —> Lemonade
hex number 15 is for mew so dec is –> 21
Lemonade x21
ld (xxyy), a is TM34 -> yy is 59 to dec is 89
TM34 x89
So then the next item describes the xx (3. Question -> is this all the time when we have a function with xxyy, that next item describes xx?) and its TM08. If i understood it right yy is read first coz of that-> part from (http://glitchcity.info/wiki/index.php/GB_Programming#A_new_world)
((Let's say the following instruction is ld ($2315), hl. Applying the same logic would mean H's value would be stored at ($2315), and L's would be at ($2316). However, you just lost THE GAME; because the z80 is a "little-endian" processor, L's value (the "little-end") is stored first, at ($2315). So ($2315) is $61, and ($2316) is $D3.))
(I think nearly the same question like 3. Question->) If we had a function like this: ld hl, (xxyy) xx would be first or not?
ret (4. Question - > i dont get it what ret is. I have never found this function on all the explanations pages i was looking^^. Does this finish the code we are writing?) but i understand with the itemgenerator page that it must be x201 for "ret".
https://eldred.fr/gbz80toitems3/ –> shows me this:
Lemonade x15 (hex:0F)
TM34 x89 (hex:59)
TM08 x201 (hex:C9)
But im confused. 5. Question -> Why we have 15 lemonade? I come to 21… ouh man…^^

#### Re: What i should note before learning ACE in german version?

Posted by: Torchickens
Date: 2019-10-10 18:56:24

> I have read now a lot of pages and all was a little bit a mess. So i started to read every single post here in the ACE Section starting by last post.
> So i came to six new questions^^.
> 1. Question:
> ISSOtm posted this page: http://glitchcity.info/wiki/index.php/GB_Programming#A_new_world
> So now im reading it and it makes a lot, really a lot clearer for me, its like i can see a little light at the end of the tunnel :).
> I have a question there about a calculation in "Register pair and RAM" section. This is a calculation result from author:
> sub a, b ; A = A - B = $06 - $DE = $06 + (-$DE) = $06 + ($21 + $01) = $28, C flag = 0
> I calculated it i think in a really "noob" Version. I calculated $06 = 6 / $DE = 222
> So 6 - 222 (and 0 is 256 / so 256 - 216) i come to 40 / $28 - okey its right.
> But i would like to understand how the writer split -$DE into $21 and $01. I tried to understand and searched above in his post, but i really dont get it.
> 2.Question:
> I have read and checked a video on youtube that a hex editor and a RAM Searcher is very good to use.
> Im not very familiar with BGB Debugger atm. Is there such a thing implemented, like a all in one solution? Or do i need RAM Searcher and hex editor seperately?
> 3. to 5. Question:
> Evie the Mother Hen has written in "Where to get started" this code:
> ld a, 15
> ld [$d059] ,a
> ret
> so atm im trying to find out by myself first which item and quantity i need and then i check with this page: https://eldred.fr/gbz80toitems3/
> So im checking in big hex list the function for ld a,x —> Lemonade
> hex number 15 is for mew so dec is –> 21
> Lemonade x21
> ld (xxyy), a is TM34 -> yy is 59 to dec is 89
> TM34 x89
> So then the next item describes the xx (3. Question -> is this all the time when we have a function with xxyy, that next item describes xx?) and its TM08. If i understood it right yy is read first coz of that-> part from (http://glitchcity.info/wiki/index.php/GB_Programming#A_new_world)
> ((Let's say the following instruction is ld ($2315), hl. Applying the same logic would mean H's value would be stored at ($2315), and L's would be at ($2316). However, you just lost THE GAME; because the z80 is a "little-endian" processor, L's value (the "little-end") is stored first, at ($2315). So ($2315) is $61, and ($2316) is $D3.))
> (I think nearly the same question like 3. Question->) If we had a function like this: ld hl, (xxyy) xx would be first or not?
> ret (4. Question - > i dont get it what ret is. I have never found this function on all the explanations pages i was looking^^. Does this finish the code we are writing?) but i understand with the itemgenerator page that it must be x201 for "ret".
> https://eldred.fr/gbz80toitems3/ –> shows me this:
> Lemonade x15 (hex:0F)
> TM34 x89 (hex:59)
> TM08 x201 (hex:C9)
> But im confused. 5. Question -> Why we have 15 lemonade? I come to 21… ouh man…^^

Hi Scroogeduck. Sure, can help: ^^
OK so
1. Without me reading/tracing ISSOtm's page in full, it seems b=$06 and a=$DE. Also that C004DE one instruction before sub a,b. But simplified, it's a b-a calculation; so if 06-DE it will underflow (you can do a modulus/Euclidean division to get this similar to your way ;- Convert to decimal number system for 6-222, get -216. Put 256 on the left side for 256-216 or 40. Convert 40 back to hexadecimal to get $28).
(There is a short paper way to convert the number systems, like as described on this webpage:
1. Divide the decimal number by 16. Treat the division as an integer division.
2. Write down the remainder (in hexadecimal).
3. Divide the result again by 16. Treat the division as an integer division.
4. Repeat step 2 and 3 until result is 0.
5. The hex value is the digit sequence of the remainders from the last to first.
However, I've usually used either a search engine for an online conversion or Windows Calculator. Setting it to programmer mode, typing a hex value then clicking dec will convert it, vice versa.
I'm unsure how he got $21 + 01 either sorry. To spread some insight, because only one byte is used, after $FF you get $00 but we can suppose this is $100. $100-$DE is $22; and $06 + $22 gives the answer but not where the $01 came from. From another approach (not sure if this is a valid proof), I also tried inversing the bits (binary representation). Type $DE into Windows Calculator on Programmer, you get 1101 1110. So 0010 0001 converted is $21. (This is the "Not" operation in electronics).
2. Sure can answer this. :) Yes with pen and paper, however BGB's debugger uses ROM banks. But in short BGB, a separate hex editor, and knowledge (or calculator) for swapping between offsets/pointers makes identifying parts of the ROM easier (sometimes the disassembly only gives pointers, other times offsets are available).
The bankless(?) or bank 0 ROM section only goes from 0000-3FFF. 4000-7FFF conversely in Pokémon is for banked ROM. These are called pointers (see also here for how the pointers map out). Pointers and offsets (the latter you'd find in many hex editors) however may not have the same definition; an offset for instance might be hex:012345 (this as a pointer is 04:6345 where 04 means the bank. As there are three byte pairs it is called a "three byte pointer"). To convert from three byte pointer to offset, you do: (Bank value)x4000 + ((Bytes after bank value) - 0x4000), which gives 012345 as expected. (Workings are 4x4000 + 6345-4000 10000 + 2345 = 12345).
For converting offset to three byte pointer, DataCrystal offers this explanation:

> Take the offset that you wish to point to.
> Take the last four digits of the offset, and cut off the rest.
> If the offset is outside the range 4000-7FFF, it must be converted like this: (offset mod 4000) + 4000. In other words:
> If the offset is from 0000-3FFF, add 4000 to the offset.
> If the offset is from 4000-7FFF, do not add anything to the offset.
> If the offset is from 8000-BFFF, subtract 4000 from the offset.
> If the offset is from C000-FFFF, subtract 8000 from the offset.
> Because Game Boy is little endian, switch the first two digits with the last two.
> Note: In rare cases, some games, such as Donkey Kong Land 2 and Donkey Kong Land III, may store pointers in big endian, in which case this step is not necessary.
> Example: $1201FA is the offset you wish to point to in a game that uses little endian pointers.
> Take the offset $1201FA
> Take the last four digits: $01FA
> Since $01FA is between 0000-3FFF, you add 4000, so you get $41FA
> Because Game Boy is little endian, you take the 41FA, switch the 41 and FA around to get FA41

May also have an alternative method?:
To convert 12345 back into 04:6345, first the last two digit pair will always be the same (for big endian); so ??:??45. Do 12345/4000 (in hexadecimal) and throw away the remainder to get 4; so 04:??45. Finally, the 63 for some reason is the "23" + 40.
Sometimes using both the debugger (which is very useful for things like breakpoints in addition to general memory viewing) and a separate hex editor is convenient. My favourite is an open source one (I believe, as of memory in writing) called "HxD", which will give you offsets for any (well maybe not strictly any but any normal allocated/on a harddrive, memory stick etc.) file. Remember too make sure read only settings are off (properties, settings, etc.). There are conveniently too calculators to switch between pointers and offsets, such as here.
3 to 5.
Yes, the items and opcodes convert :). It depends really where the code is in RAM. If it's executing say D327 (item 3 in German/French/Italian/Spanish Red) then the ld a, part (for this opcode) is Lemonade but as D328 is item 3 quantity, the $15 is the x21. The reason why it's x21 is because the item quantities appear in decimal in the game, and the hexadecimal $15 becomes 21 in decimal (or 16*1 [left most digit]+5 [right most]). For the ld (yyxx), a (for this opcode TM34) after, we're at D329 (item 4), then D32A (item 4 quantity is $59 - in game; this is decimal:89) and this pattern continues.
Note you can use normally unnecessary padding to switch between whether it becomes an item or quantity. So, for instance using Poké Ball (inc b) straight after the ld a,$15 in the above would mean Poké Ball x 234 (EA in decimal is 234) instead of a TM34 stack.
Additionally I may have forgotten to consider the D059 in German version would be D05E (+5 of original) instead, so a quantity of 94, sorry. :(
yyxx instead of xxyy means what is called little endian; so 5ED0 (for yyxx) instead of D05E (xxyy) etc.
For your following similar question, if the documentation said something like ld hl, (xxyy) then it may be 21 D0 5E or either: Thunderstone x 208 and 9F, or (item) x 33 [21 in dec] and TM08 x 94, however the available opcode in this case is ld hl, (yyxx) so it would be 21 5E D0: Thunderstone x 94 and TM08 or (item) x 33 [21 in dec] 9F x 208.
Finally, ret is normally needed to end the code without a freeze. Basically (shorter answer) it can be understood like 'make the game return to what it should be doing after the arbitrary code execution finishes' (so return to item menu processing, music processing, etc.), with the absence of it causing the game to execute code in the next memory addresses indefinitely (often with this the game runs rst $38 [opcode $FF at some point] which in Pokémon Gen I runs a 0038 containing another $FF, which is an endless loop and responsible for the 00 39 corruption freezes). The longer answer is "Pop two bytes from stack & jump to that address." where the stack pointer (imagine like a stack of papers with each one a command) and pop, push instructions are involved. I've forgotten or never fully registered which of these (pop and push) do what, but this can likely be found with BGB debugger experimentation. Ret will not work if the ACE method (or part of the code) corrupts the stack without fixing it. Some methods (like Coin Case glitch in Generation II) do break the stack so have workarounds included in the codes, but usually you can use ret (TM01 or quantity 201) at the end of your code and don't have to worry about how to use it.
Hope this helps. ^^

#### Re: What i should note before learning ACE in german version?

Posted by: Scroogeduck
Date: 2019-10-11 07:54:36
Hey Evie the Mother Hen thank you very much for your help and the long answers :).

> (There is a short paper way to convert the number systems, like as described on this webpage:
> 1. Divide the decimal number by 16. Treat the division as an integer division.
> 2. Write down the remainder (in hexadecimal).
> 3. Divide the result again by 16. Treat the division as an integer division.
> 4. Repeat step 2 and 3 until result is 0.
> 5. The hex value is the digit sequence of the remainders from the last to first.
> However, I've usually used either a search engine for an online conversion or Windows Calculator. Setting it to programmer mode, typing a hex value then clicking dec will convert it, vice versa.
> I'm unsure how he got $21 + 01 either sorry. To spread some insight, because only one byte is used, after $FF you get $00 but we can suppose this is $100. $100-$DE is $22; and $06 + $22 gives the answer but not where the $01 came from. From another approach (not sure if this is a valid proof), I also tried inversing the bits (binary representation). Type $DE into Windows Calculator on Programmer, you get 1101 1110. So 0010 0001 converted is $21. (This is the "Not" operation in electronics).

Thanks for the page and the instruction. That is a very nice option to calculate.
No problem coz of +$01, if this is not really necessary to know and the version to calculate from hex - dec and back to hex is working good enough so its everything allright :) - thanks.
I thought maybe its important to know how he did it to follow the things later…

> 2. Sure can answer this. :) Yes with pen and paper, however BGB's debugger uses ROM banks. But in short BGB, a separate hex editor, and knowledge (or calculator) for swapping between offsets/pointers makes identifying parts of the ROM easier (sometimes the disassembly only gives pointers, other times offsets are available).
> The bankless(?) or bank 0 ROM section only goes from 0000-3FFF. 4000-7FFF conversely in Pokémon is for banked ROM. These are called pointers (see also here for how the pointers map out). Pointers and offsets (the latter you'd find in many hex editors) however may not have the same definition; an offset for instance might be hex:012345 (this as a pointer is 04:6345 where 04 means the bank. As there are three byte pairs it is called a "three byte pointer"). To convert from three byte pointer to offset, you do: (Bank value)x4000 + ((Bytes after bank value) - 0x4000), which gives 012345 as expected. (Workings are 4x4000 + 6345-4000 10000 + 2345 = 12345).
> For converting offset to three byte pointer, DataCrystal offers this explanation:

> > Take the offset that you wish to point to.
> > Take the last four digits of the offset, and cut off the rest.
> > If the offset is outside the range 4000-7FFF, it must be converted like this: (offset mod 4000) + 4000. In other words:
> > If the offset is from 0000-3FFF, add 4000 to the offset.
> > If the offset is from 4000-7FFF, do not add anything to the offset.
> > If the offset is from 8000-BFFF, subtract 4000 from the offset.
> > If the offset is from C000-FFFF, subtract 8000 from the offset.
> > Because Game Boy is little endian, switch the first two digits with the last two.
> > Note: In rare cases, some games, such as Donkey Kong Land 2 and Donkey Kong Land III, may store pointers in big endian, in which case this step is not necessary.
> > Example: $1201FA is the offset you wish to point to in a game that uses little endian pointers.
> > Take the offset $1201FA
> > Take the last four digits: $01FA
> > Since $01FA is between 0000-3FFF, you add 4000, so you get $41FA
> > Because Game Boy is little endian, you take the 41FA, switch the 41 and FA around to get FA41

> May also have an alternative method?:
> To convert 12345 back into 04:6345, first the last two digit pair will always be the same (for big endian); so ??:??45. Do 12345/4000 (in hexadecimal) and throw away the remainder to get 4; so 04:??45. Finally, the 63 for some reason is the "23" + 40.
> Sometimes using both the debugger (which is very useful for things like breakpoints in addition to general memory viewing) and a separate hex editor is convenient. My favourite is an open source one (I believe, as of memory in writing) called "HxD", which will give you offsets for any (well maybe not strictly any but any normal allocated/on a harddrive, memory stick etc.) file. Remember too make sure read only settings are off (properties, settings, etc.). There are conveniently too calculators to switch between pointers and offsets, such as here

Okey, I definetely have to read this part more times, i dont get it all right now. That goes to a file you posted one time right, the sym file?
I will use hex editor and play a little bit with it… maybe it helps too :).

> 3 to 5.
> Yes, the items and opcodes convert :). It depends really where the code is in RAM. If it's executing say D327 (item 3 in German/French/Italian/Spanish Red) then the ld a, part (for this opcode) is Lemonade but as D328 is item 3 quantity, the $15 is the x21. The reason why it's x21 is because the item quantities appear in decimal in the game, and the hexadecimal $15 becomes 21 in decimal (or 16*1 [left most digit]+5 [right most]). For the ld (yyxx), a (for this opcode TM34) after, we're at D329 (item 4), then D32A (item 4 quantity is $59 - in game; this is decimal:89) and this pattern continues.
> Note you can use normally unnecessary padding to switch between whether it becomes an item or quantity. So, for instance using Poké Ball (inc b) straight after the ld a,$15 in the above would mean Poké Ball x 234 (EA in decimal is 234) instead of a TM34 stack.

Ouh man dude, that makes a lot clearer. I didnt know that it goes from like in this example from D327 to D328 to D329 and so on… I often thought about the problem why we know where the next thing goes and i didnt understand it^^. That was a very cool information, thanks :).

> Additionally I may have forgotten to consider the D059 in German version would be D05E (+5 of original) instead, so a quantity of 94, sorry. :(
> yyxx instead of xxyy means what is called little endian; so 5ED0 (for yyxx) instead of D05E (xxyy) etc.
> For your following similar question, if the documentation said something like ld hl, (xxyy) then it may be 21 D0 5E or either: Thunderstone x 208 and 9F, or (item) x 33 [21 in dec] and TM08 x 94, however the available opcode in this case is ld hl, (yyxx) so it would be 21 5E D0: Thunderstone x 94 and TM08 or (item) x 33 [21 in dec] 9F x 208.

Coz of german version D05E no problem dude :). Its absolutely okey that i have to think about that and understand it.
That makes it clearer too.

> Finally, ret is normally needed to end the code without a freeze. Basically (shorter answer) it can be understood like 'make the game return to what it should be doing after the arbitrary code execution finishes' (so return to item menu processing, music processing, etc.), with the absence of it causing the game to execute code in the next memory addresses indefinitely (often with this the game runs rst $38 [opcode $FF at some point] which in Pokémon Gen I runs a 0038 containing another $FF, which is an endless loop and responsible for the 00 39 corruption freezes). The longer answer is "Pop two bytes from stack & jump to that address." where the stack pointer (imagine like a stack of papers with each one a command) and pop, push instructions are involved. I've forgotten or never fully registered which of these (pop and push) do what, but this can likely be found with BGB debugger experimentation. Ret will not work if the ACE method (or part of the code) corrupts the stack without fixing it. Some methods (like Coin Case glitch in Generation II) do break the stack so have workarounds included in the codes, but usually you can use ret (TM01 or quantity 201) at the end of your code and don't have to worry about how to use it.
> Hope this helps. ^^

Just interested in the ret function. Is there a difference between the rets like "ret nz" "ret z"… I think that it has a difference, but do i only need "ret" (TM01/ or x201) and the others i can forget :)?
Yep, it helped a lot :).
Thanks again a lot for your good answers and explanations :).
Have a nice day

#### Re: What i should note before learning ACE in german version?

Posted by: Couldntthinkofaname
Date: 2019-10-11 11:03:19

> Just interested in the ret function. Is there a difference between the rets like "ret nz" "ret z"… I think that it has a difference, but do i only need "ret" (TM01/ or x201) and the others i can forget :)?
> Yep, it helped a lot :).
> Thanks again a lot for your good answers and explanations :).
> Have a nice day

"ret" is a opcode that pops the value of the top of the stack and jumps to it,and it's responsible for returning from functions.
The Zero Flag (Z) is a CPU flag that is "set" by any operation (with the exception of loading) that causes the value that the opcode was working with to be zero
For Example:
`
ld a,$03; the accumulator is now 3
sub $03 ; the accumulator is now 0 and the Zero flag is set.
`
Loading does not set the zero flag
`
ld a,0 ; zero flag not set
and a ; ...now it is!
`
Opcodes that use "nz" and "z" check the zero if is it set, and perform operations depending on whether or not it is
Example:
`
start:
ld a,$03
sub $03
jr z, .skip
call doSomething ; THIS IS NEVER CALLED!
.skip:
call doOtherStuff
`
`
start:
ld a,$03
sub $03
jr nz, .skip
call doSomething ; this is called
.skip:
call doOtherStuff
`
Similarly, "ret nz" returns if the zero flag is not set, and "ret z" returns if it is. "ret" returns regardless

#### Re: What i should note before learning ACE in german version?

Posted by: Scroogeduck
Date: 2019-10-12 11:33:13

> "ret" is a opcode that pops the value of the top of the stack and jumps to it,and it's responsible for returning from functions.
> The Zero Flag (Z) is a CPU flag that is "set" by any operation (with the exception of loading) that causes the value that the opcode was working with to be zero
> For Example:
> `
> ld a,$03; the accumulator is now 3
> sub $03 ; the accumulator is now 0 and the Zero flag is set.
> `
> Loading does not set the zero flag
> `
> ld a,0 ; zero flag not set
> and a ; ...now it is!
> `
> Opcodes that use "nz" and "z" check the zero if is it set, and perform operations depending on whether or not it is
> Example:
> `
> start:
> ld a,$03
> sub $03
> jr z, .skip
> call doSomething ; THIS IS NEVER CALLED!
> .skip:
> call doOtherStuff
> `
> `
> start:
> ld a,$03
> sub $03
> jr nz, .skip
> call doSomething ; this is called
> .skip:
> call doOtherStuff
> `
> Similarly, "ret nz" returns if the zero flag is not set, and "ret z" returns if it is. "ret" returns regardless

Hello Epsilon
Thanks for your answers and the explanations. Yep now i understand it, thanks :).
Have a nice weekend
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