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Flashing Roms and partitions (A-B)

OK so now that Roms are officially being released for our device, even with a RC TWRP. I am wondering how and if I have to flash a bootloader to my non primary partition (B) or if I can install TWRP to main partion A and then install Carbon worried before I do it would prefer to not brick phone, thanks in advance guys

ne0ns4l4m4nder said:
OK so now that Roms are officially being released for our device, even with a RC TWRP. I am wondering how and if I have to flash a bootloader to my non primary partition (B) or if I can install TWRP to main partion A and then install Carbon worried before I do it would prefer to not brick phone, thanks in advance guys
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When you install TWRP it installs to both partitions. Both partitions should have a bootloader on them already, I would however flash a factory image first them flash twrp, rom etc. The rom will install to the opposite partitions that you are currently on.

Many questions on the A/B slots Which caused a fastboot bootloop

Man,i hate that oneplus did this with the A/B slots..
Had successfully unlocked the bootloader,installed twrp and had tryed the few roms that are out for the oneplus 6..with no problems..
Then,i thought i'd make a backup of my data from my oneplus 3T and restore that to my oneplus 6 (Big mistake)
At first it would bootloop back into twrp,so i did a system and data wipe,and tryed flashing a rom.before flashing i checked to see which slot i was in,and it would say "A"
But after flashing the rom and twrp,and booting back into twrp,it says slot "B" was active and no OS was on slot "B".I tryed flashing the rom to both "A" and "B" slot,then it just bootlooped back into fastboot mode..Thanks to whoever uploaded that unbrick tool,so now i'm back to stock,with a locked bootloader.I really don't understand this A/B slot thing and how it works..
1)Does the stock OS reside in one of the slots all the time?Or when you flash a rom,does it install to both the slots?
2)Do you have to always make sure you only flash to one certain slot?
3)When you do a system and data wipe,does it also wipe the vendor.img on both slots?
Any help would be greatly appreciated..Thanks

Wäre auch für mich interessant.

saleappeal said:
Wäre auch für mich interessant.
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English only noob.

bobbylo said:
Man,i hate that oneplus did this with the A/B slots..
Had successfully unlocked the bootloader,installed twrp and had tryed the few roms that are out for the oneplus 6..with no problems..
Then,i thought i'd make a backup of my data from my oneplus 3T and restore that to my oneplus 6 (Big mistake)
At first it would bootloop back into twrp,so i did a system and data wipe,and tryed flashing a rom.before flashing i checked to see which slot i was in,and it would say "A"
But after flashing the rom and twrp,and booting back into twrp,it says slot "B" was active and no OS was on slot "B".I tryed flashing the rom to both "A" and "B" slot,then it just bootlooped back into fastboot mode..Thanks to whoever uploaded that unbrick tool,so now i'm back to stock,with a locked bootloader.I really don't understand this A/B slot thing and how it works..
1)Does the stock OS reside in one of the slots all the time?Or when you flash a rom,does it install to both the slots?
2)Do you have to always make sure you only flash to one certain slot?
3)When you do a system and data wipe,does it also wipe the vendor.img on both slots?
Any help would be greatly appreciated..Thanks
Click to expand...
Click to collapse
Ok let me answer your three questions here.
1. The rom resides on both partitions. The whole point of A/B partitions is seamless updates. It let you use your phone on one slot while the update downloads and updates on the other slot. Then after the update when you restart the phone it boots up the other slot (the one that updated) and you are running the updated software. It also checks the update and if taking the update causes a bootloop it reloads the old slot so you don't end up bricked from a bad OTA update.
2. When flashing a new custom rom meant for this device, it should flash it to both slots.
3. Vendor partition is separate now (thank you project treble) from system and data. Wiping system and data should never affect the vendor partition.
Any other questions please feel free to ask. I would rather answer questions this way than dealing with a panicking person who bricked their phone and can't unbrick it.

tabletalker7 said:
Ok let me answer your three questions here.
1. The rom resides on both partitions. The whole point of A/B partitions is seamless updates. It let you use your phone on one slot while the update downloads and updates on the other slot. Then after the update when you restart the phone it boots up the other slot (the one that updated) and you are running the updated software. It also checks the update and if taking the update causes a bootloop it reloads the old slot so you don't end up bricked from a bad OTA update.
2. When flashing a new custom rom meant for this device, it should flash it to both slots.
3. Vendor partition is separate now (thank you project treble) from system and data. Wiping system and data should never affect the vendor partition.
Any other questions please feel free to ask. I would rather answer questions this way than dealing with a panicking person who bricked their phone and can't unbrick it.
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I’m sorry i don’t want to hijack this thread but i want to make a Nandroid backup in TWRP and saw an option to choose from slot A or B.. my question is: Is there any difference between those two or does it matter what slot you are backing up in? Edit: i looked on google and could't find any information about this.

jdorp18 said:
I’m sorry i don’t want to hijack this thread but i want to make a Nandroid backup in TWRP and saw an option to choose from slot A or B.. my question is: Is there any difference between those two or does it matter what slot you are backing up in? Edit: i looked on google and could't find any information about this.
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It doesn't matter what slot you are on you will be making the backup of the current slot by default, and that is exactly what you want

Everything you need to know is here:
https://forum.xda-developers.com/oneplus-6/how-to/guide-noobs-guide-to-b-partitions-op6-t3816123
In most cases you should not worry which slot you're on, just let the system take care of that. On some custom roms, it matters, and in those cases carefully follow the installation instructions.

Thanks guys, this answers my question. (Edit: I've just read the Generic Noob Advice. i don't think i can delete this comment.)

A/B slot issue, TWRP only on one... unrooted rom on other. Can't mount from TWRP

TWRP is on slot B but not slot A. I can only reboot to system from TWRP when I've got in on Slot A so my ROM seems to be on A,
But from TWRP I can't see the storage device, it says 0MB. It won't let me mount. From what I gather, this is because it's failing to decrypt the device from TWRP? I trying resetting my pin but it didn't work.
I'm trying to figure out how to root my slot A image that doesn't seem to be rooted anymore after an update. How can I get TWRP from Slot B to see the device?
I'm a bit confused about the whole A/B images. What is the function of having two images, besides one being the backup?
Thanks for any suggestions anyone might have.

el80ne said:
I'm trying to figure out how to root my slot A image that doesn't seem to be rooted anymore after an update. How can I get TWRP from Slot B to see the device?
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So it sounds like slot A is booting to OS (and the updated OS version)?
My suggestion would be to: on slot A, fastboot boot TWRP img, and flash TWRP installer zip again. Then (still booted TWRP) flash Magisk.
Make sure it is a recent version TWRP (latest is unofficial version -76), as it seems that some of the data partition issues are related to using old TWRP.
el80ne said:
I'm a bit confused about the whole A/B images. What is the function of having two images, besides one being the backup?
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Click to collapse
Yeah, the A/B partition scheme was new to me for this device as well (and lots of other folks).
The main purpose is to have what Google calls "seamless updates". Where on a stock device, it is just installing the update in the background and you can still use the phone as it updates. Then once the update is complete, just reboot, and you are on the new OS version. And yes, if something goes wrong, it still has the other (not updated) slot to fall back to. The following xda article is a good read, and explains the basic premise of A/B partitions:
https://www.xda-developers.com/how-...ess-updates-affect-custom-development-on-xda/
The A/B partitions do makes things more complicated when modifying/rooting, etc. But there is a pretty easy way to do an OTA update on this phone, and keep TWRP/root. But let's get the current situation fixed up, before I confuse you with other scenarios.

A/B slot questions

I tend to mess things up when flashing new ROMs on my Oneplus6. And I usually end up using the MSMdownloadtool to 'fix my phone' again.
* Does MSMdownloadtool install OOS on both slots?
* What do you advise to do before installing a custom (v10) rom?
* I tried doing a fastboot OOS install by mauronofrio, it didnt work. I got into bootloops after running/finishing the flash all bat file. So I used MSMdownloadtool again. I wondered if the fastboot OOS installer guide by mauronofrio installs OOS on both slots??
* Does the fastboot OOS installer do the same thing as MSMdownloadtool?
* In a lot of guides for clean install custom rom I read as first step: wipe system/data. But then I wonder why do you have to install the correct OOS version first, THEN wipe it, and then install custom ROM. Does an OOS installation leave some hidden (essential) files/partitions behind or something?
* Do you guys switch slots in twrp / or do you reboot to switch slots / or do you use the fastboot --set-active a/b command?
* Why does gapps only have to be installed on 1 slot and not on both?
* Why don't custom ROMS install themselves on both slots straight away, like when you flash the twrp installer zip.. ?
Im currently running carbonROMv8 ROM, its running fine so Im not looking to change yet. But im sure im gonna have to use msmdowndtool again when I wanna try out some other ROM in the future. I dont really feel like im having any control off the flashing process. It either works or it doesnt and when it doesnt I cant explain why..

dumbl3 said:
I tend to mess things up when flashing new ROMs on my Oneplus6. And I usually end up using the MSMdownloadtool to 'fix my phone' again.
* Does MSMdownloadtool install OOS on both slots?
* What do you advise to do before installing a custom (v10) rom?
* I tried doing a fastboot OOS install by mauronofrio, it didnt work. I got into bootloops after running/finishing the flash all bat file. So I used MSMdownloadtool again. I wondered if the fastboot OOS installer guide by mauronofrio installs OOS on both slots??
* Does the fastboot OOS installer do the same thing as MSMdownloadtool?
* In a lot of guides for clean install custom rom I read as first step: wipe system/data. But then I wonder why do you have to install the correct OOS version first, THEN wipe it, and then install custom ROM. Does an OOS installation leave some hidden (essential) files/partitions behind or something?
* Do you guys switch slots in twrp / or do you reboot to switch slots / or do you use the fastboot --set-active a/b command?
* Why does gapps only have to be installed on 1 slot and not on both?
* Why don't custom ROMS install themselves on both slots straight away, like when you flash the twrp installer zip.. ?
Im currently running carbonROMv8 ROM, its running fine so Im not looking to change yet. But im sure im gonna have to use msmdowndtool again when I wanna try out some other ROM in the future. I dont really feel like im having any control off the flashing process. It either works or it doesnt and when it doesnt I cant explain why..
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Yoo.. So Many Questions. Will try to explain in easiest way..
1. Yes. It Does Flash OOS in Both Slot via Qualcomm Download Mode which is mainly for Manufactures to restore Device..
2. Let me clear that Most people's who face issue in booting custom ROM have their data encrypted by previous or stock ROM.
Here is Link Thread to know how to decrypt your device..
https://forum.xda-developers.com/oneplus-6/how-to/tutorial-decrypt-flash-rom-pie-oreo-roms-t3838643
My own Instructions to Flash Custom ROM (Strictly for Decrypted Device):-
Boot Into Recovery
Wipe Cache, Data & System
Flash Latest Stable/Beta OOS10 & Recovery
Reboot Into Recovery(Ignore No OS Installed Warning)
Flash Latest Stable/Beta OOS10 & Recovery again
Reboot Into Recovery
Now flash Custom ROM + Recovery(If Not Included)
Reboot Into Recovery
Flash Gapps(If not included in ROM), Magisk & No-Verity zip(to keep Device dencrypted)
Reboot & Done
For Installing Update on Same Custom ROM :-
Download the Update
Reboot Into Recovery
Flash Update ZIP & Recovery
Reboot Into Recovery
Flash Gapps(If not included in ROM), Magisk & No-Verity zip(to keep Device dencrypted)
Reboot & Done
3. Fastboot Don't always work if You try to Upgrade/Downgrade Android Version as it won't flash Critical Partitions.. Meanwhile MSMDownload tool Use Download mode which is more powerful.
4. Nope. Both are Different AS both use different mode to flash ROM.
5. Clean install required while flashing new custom ROM because Not All ROM uses Same Base. Some uses Stable OOS or some uses Beta OOS... Now, your answer is in these two build Types...
In my Opinion, Custom ROM have nothing to do with your System partition but it most like to related to Vendor Partition... Both Stable & Beta OOS Have little difference in Vendor partition data..
When any dev Build Any ROM, he uses any one of those Build as base for which he tweak/fix bugs.. That's why always Use Dev specified OOS build for avoid Bootloop/bugs..
6. I prefer last option as it's more reliable..
7. It's like when you clean install any ROM.. When you install Any ZIP, It get installed in an Inactive Slot which make those ZIP data Allowed to use in Active slot..
But when you update ROM for 1st time, You install Gapps as well.. So i don't think you need to Install Gapps on 2nd Update as it already installed in both slots.
1st time when Clean installed & 2nd time when Updated 1st time...
8. It's Because how A/B Partition works... Any zip you flash get installed in inactive slot & this applies to both Stock OOS & Custom ROM...
That's why we have to flash OOS twice before flashing any custom ROM.. Which confirms you will have same Vendor partition regardless to your current slot so when you flash custom ROM it will get booted anyways even after an update..
Hope that's helpful.. Any question or help, you can DM or reply here..

Thanks for your reply. It clears a few things up I think. I got a few follow up questions.
I don't really feel comfortable walking around with a decrypted phone. I have already been pick-pocketed once and also lost two smartphones. I would say the chances are likely that also my OP6 is gonna get lost at some point. Having it decrypted would leave it very vulnerable for people trying to access my files.
* So lets say I decrypt my phone. Is it possible then to encrypt the phone again after I have flashed a new ROM (for example LineageOS 17.1)?

dumbl3 said:
Thanks for your reply. It clears a few things up I think. I got a few follow up questions.
I don't really feel comfortable walking around with a decrypted phone. I have already been pick-pocketed once and also lost two smartphones. I would say the chances are likely that also my OP6 is gonna get lost at some point. Having it decrypted would leave it very vulnerable for people trying to access my files.
* So lets say I decrypt my phone. Is it possible then to encrypt the phone again after I have flashed a new ROM (for example LineageOS 17.1)?
Click to expand...
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Well, In my Opinion Every Device is Vulnerable to Hackers.. Doesn't matter if it's bootloader is Locked or Unlocked...
Your device is more Vulnerable at the time you unlocked bootloader because after that you can execute advanced commands with fastboot.....
If your device have pin/pattern, Thief gone reset your device anyway to reset pass.. Eventually your data get wiped as well....
Well, you can encrypt your data but you might face issue while migrating to other ROM if you face any bug + decrypting your encrypted device again is time taking process if you have alot of data..

I mean there is difference between someone being able to reset/delete my data than someone being able to read my data. As far as I know a thief cannot just read my data after he has stolen my device which is encrypted with unlocked bootloader. Sure, he can connect it to his computer and fastboot delete my sh!t but I don't care I got backups of my most important data anyway, as long as he/she cannot just read my device data its fine by me. But how would I encrypt my device again after I installed, lets say, LineageOS 17.1?

dumbl3 said:
I mean there is difference between someone being able to reset/delete my data than someone being able to read my data. As far as I know a thief cannot just read my data after he has stolen my device which is encrypted with unlocked bootloader. Sure, he can connect it to his computer and fastboot delete my sh!t but I don't care I got backups of my most important data anyway, as long as he/she cannot just read my device data its fine by me. But how would I encrypt my device again after I installed, lets say, LineageOS 17.1?
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you can.. But do it only if you gone use it for long term....
Not fine to waste your whole day transfer your date from mobile to PC then PC to mobile..

how to know if it's a bootloop?

hello i was upgrading from stock 9.0.7 to 9.0.9, my bootloader is unlocked and twrp is a permanent one, the phone is also rooted. i wiped system then flashed the 9.0.9 then flashed the 4.0.4 twrp, rebooted into twrp, flashed latest magisk, rebooted into twrp and then pressed reboot into system. the phone is encrypted. the booting animation has been on for 20-30 minutes at this point. is it a bootloop? why what did i do wrong? thanks

@seth wellington
Try formatting data them reboot system. That should get you booted up.
Never wipe system or vendor on these devices and always when flashing a rom flash TWRP installer zip or you won't have twrp because of where recovery is located which is same place as boot.img and it has to be installed when flashing a rom or update to a rom or it won't be present.
How I flash OOS on my once T-Mobile now converted international/global OnePlus 6t.
Boot TWRP
Flash OOS global 10.3.6
Flash TWRP installer
Flash finalize
Format data
Reboot system
How I install a rom on my converted OnePlus 6t. This installation process varies depending on whether or not whatever rom I'm installing comes with gapps and or TWRP and of course you don't want to install global OOS on another model device. Since I converted my T-Mobile OnePlus 6t using guide and msm tool I now use global or international OnePlus 6t roms, firmware... Example Pixel Experience rom installation:
Boot TWRP
Flash OOS 10.3.6 global from repo xda thread
Flash Pixel Experience rom
Flash TWRP installer zip
Flash finalize zip
Format data
Reboot system.
Start to set up device then at some point enable developer options and usb debugging and doable any lock screen and boot twrp and flash magisk for root and reboot system. That is how I install a rom that has gapps but not twrp included in rom zip. Finalize can he found with a quick search on xda. It copies firmware to other slot on your AB partition device so that you don't have to. Saves 5-6 minutes and works great.
Edit
What you could have done is just flash your updated OOS , flash TWRP installer zip, reboot recovery, flash magisk, reboot system. That's what I would have done updating OOS ..
Hope this helps.
Sent from my OnePlus6T using XDA Labs

flash713 said:
@seth wellington
Try formatting data them reboot system. That should get you booted up.
Never wipe system or vendor on these devices and always when flashing a rom flash TWRP installer zip or you won't have twrp because of where recovery is located which is same place as boot.img and it has to be installed when flashing a rom or update to a rom or it won't be present.
How I flash OOS on my once T-Mobile now converted international/global OnePlus 6t.
Boot TWRP
Flash OOS global 10.3.6
Flash TWRP installer
Flash finalize
Format data
Reboot system
How I install a rom on my converted OnePlus 6t. This installation process varies depending on whether or not whatever rom I'm installing comes with gapps and or TWRP and of course you don't want to install global OOS on another model device. Since I converted my T-Mobile OnePlus 6t using guide and msm tool I now use global or international OnePlus 6t roms, firmware... Example Pixel Experience rom installation:
Boot TWRP
Flash OOS 10.3.6 global from repo xda thread
Flash Pixel Experience rom
Flash TWRP installer zip
Flash finalize zip
Format data
Reboot system.
Start to set up device then at some point enable developer options and usb debugging and doable any lock screen and boot twrp and flash magisk for root and reboot system. That is how I install a rom that has gapps but not twrp included in rom zip. Finalize can he found with a quick search on xda. It copies firmware to other slot on your AB partition device so that you don't have to. Saves 5-6 minutes and works great.
Edit
What you could have done is just flash your updated OOS , flash TWRP installer zip, reboot recovery, flash magisk, reboot system. That's what I would have done updating OOS ..
Hope this helps.
Sent from my OnePlus6T using XDA Labs
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Thank you very much for your reply. Unfortunately I couldn't wait for that long and I had to do something right then. After some attempts I also lost my twrp and was left without a functioning recovery, so I had to look for help and some people helped me go through unbricking through the msm tool.
The thing is i just really want to always flash clean so that the phone doesn't lag and works properly, I had no idea you shouldn't wipe system... The guide that I followed said that when updating oxygen manually you have to wipe system so I thought nothing would go wrong. I still don't know what caused a bootloop since that guide was a verified one. Thanks a lot for your help. So if I'm reading it correctly, you have to do the wipes after you've installed the system, not before?

seth wellington said:
Thank you very much for your reply. Unfortunately I couldn't wait for that long and I had to do something right then. After some attempts I also lost my twrp and was left without a functioning recovery, so I had to look for help and some people helped me go through unbricking through the msm tool.
The thing is i just really want to always flash clean so that the phone doesn't lag and works properly, I had no idea you shouldn't wipe system... The guide that I followed said that when updating oxygen manually you have to wipe system so I thought nothing would go wrong. I still don't know what caused a bootloop since that guide was a verified one. Thanks a lot for your help. So if I'm reading it correctly, you have to do the wipes after you've installed the system, not before?
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Click to collapse
Quit a few pretty well known developers here on xda (AOSiP op is one and there are more) say not to wipe system or vendor in their first posts of their custom rom threads. The way I describe how I install works 100% always and is the cleanest possible installation. Formatting data when installing a new rom at the end of install process does all the wiping needed to make sure nothing is left from the past and everything is clean. I used to wipe various partitions but on other devices. The install directions I use are originally from the PiXeN rom developers op. I've been flashing roms since the Nexus S days. Not saying that means anything other than I'm kinda old now. ? But that install process has never failed me once and I've flashed I'm sure hundreds of times on this device.. When I update i just flashing the updated rom over current and don't wipe anything then flashing twrp installer zip, reboot recovery flash magisk reboot system or if gapps are needed when I reboot recovery flash gapps..Unless it's a major OS update that's always worked for me.
Possibly not formatting data caused boot loop.. I know when going from OOS to AOSP rom format data is mandatory or it won't boot system.. I never flash magisk during initial flash. I always boot system first then at some point boot twrp and flash magisk. Try that next round and see if it turns out better. The whole AB partition deal made me feel like I was really old and retarded.
If I was updating OOS I would boot recovery, flash OOS, flash twrp installer zip reboot recovery, flash magisk, reboot system. Check out the finalize script xda thread. There are two here. The original person who made finalize has a thread and another user has another finalize thread. I believe in guides section of OnePlus 6t. With Android there are always more than a few ways to achieve something you know ... But what I post here is what I've been doing and I've flashed 400-500 times at least on this device.
Sent from my OnePlus6T using XDA Labs

@flash713
I totally trust you and respect anything you say and recommend me to do. Thanks a lot for all the instructions and clarification. I've been into android far less time than you — only since oneplus one. I'm extremely bad with tech but even with that phone j somehow managed to get bootloops or half-bricks ten times less than with the oneplus 6. And back then in 2015 I was way more enthusiastic: flashing twice a month loads and loads of roms. Of course it wasn't my personal achievement, I was still simply following detailed and carefully written instructions on forums on how to flash a particular rom in a particular way, but hey, at least I somehow managed it.
Now in this case I'm assuming it's the a/b partitions they've introduced that have completely changed the process and ways of flashing to the point that I stick to the guides and still get bootloops lol. That unbricking process was a huge stress for me, I don't think I'm going to touch anything in my phone for a while now. And I'm on the latest 10.3.6 oxygen so it's alright. Thank you for your help buddy

seth wellington said:
@flash713
I totally trust you and respect anything you say and recommend me to do. Thanks a lot for all the instructions and clarification. I've been into android far less time than you — only since oneplus one. I'm extremely bad with tech but even with that phone j somehow managed to get bootloops or half-bricks ten times less than with the oneplus 6. And back then in 2015 I was way more enthusiastic: flashing twice a month loads and loads of roms. Of course it wasn't my personal achievement, I was still simply following detailed and carefully written instructions on forums on how to flash a particular rom in a particular way, but hey, at least I somehow managed it.
Now in this case I'm assuming it's the a/b partitions they've introduced that have completely changed the process and ways of flashing to the point that I stick to the guides and still get bootloops lol. That unbricking process was a huge stress for me, I don't think I'm going to touch anything in my phone for a while now. And I'm on the latest 10.3.6 oxygen so it's alright. Thank you for your help buddy
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Np. My real first name is Seth. Haha. Cool name btw. I just installed OOS 10.3.7 on my 6t. I was notified of the update yesterday via oxygen updater app from play store. Good handy app to have. I would have never known 10.3.7 was out or at least not so soon if it wasn't for that app. I believe it auto notified me too. I don't remember setting it up. Android has changed soooo much you know.. I love my OnePlus 6t though. Definitely one of my most favorite devices I've ever used. What's new today is gone at some point and replaced with "better.." lol. Not always better imo anyway. Glad I could help. Feel free to message me any time. Good day to you.