DOS ain't dead - Cannot boot DOS on ASRock B850M Pro with CMS from USB

RayeR

CZ,
03.09.2026, 00:23

Cannot boot DOS on ASRock B850M Pro with CMS from USB (Users)

I had a chance to play for a moment with relative new PC at work ASRock B850M Pro with AMD Ryzen 7 8700G with Radeon 780M IGPU so I tried just boot DOS from USB stick. I still can find CSM enable option in SETUP but it didn't work. I just got a black screen with white cursor on top (non-blinking). When CSM enabled there appeared another option: Launch video OpROM Policy: UEFI only|Legacy Only|Do not launch. So I changed it to Legacy Only but it hanged during POST with a black screen I even cannot enter SETUP anymore - have to reset CMOS. Then I read that AM5 CPU's IGPU completly dropped legacy VGA BIOS so no chance. As last resort I tried to put there an old PCIe GeForce 730 but still cannot boot DOS, Legacy Only option behaved the same. In UEFI mode I can boot UEFI shell and other stuff from that USB stick. So beware that CSM option doesn't always mean it could boot DOS...

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DOS gives me freedom to unlimited HW access.

FFK

03.09.2026, 11:30

@ RayeR

Cannot boot DOS on ASRock B850M Pro with CMS from USB

Post reply

> I had a chance to play for a moment with relative new PC at work ASRock
> B850M Pro with AMD Ryzen 7 8700G with Radeon 780M IGPU so I tried just boot
> DOS from USB stick. I still can find CSM enable option in SETUP but it
> didn't work. I just got a black screen with white cursor on top
> (non-blinking). When CSM enabled there appeared another option: Launch
> video OpROM Policy: UEFI only|Legacy Only|Do not launch. So I changed it to
> Legacy Only but it hanged during POST with a black screen I even cannot
> enter SETUP anymore - have to reset CMOS. Then I read that AM5 CPU's IGPU
> completly dropped legacy VGA BIOS so no chance. As last resort I tried to
> put there an old PCIe GeForce 730 but still cannot boot DOS, Legacy Only
> option behaved the same. In UEFI mode I can boot UEFI shell and other stuff
> from that USB stick. So beware that CSM option doesn't always mean it could
> boot DOS...

my ASUS PRIME B650M-A II-CSM / Ryzen 7700x boots without problems thanks to CSMWRAP. As said, there is no VGA compatibility, only Apps using VESA will work.

RayeR

CZ,
03.09.2026, 14:21

@ FFK

Cannot boot DOS on ASRock B850M Pro with CMS from USB

Post reply

Did you tried with dedicated VGA with built-in CSM without CSMWRAP?

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DOS gives me freedom to unlimited HW access.

FFK 03.09.2026, 17:16 @ RayeR	Cannot boot DOS on ASRock B850M Pro with CMS from USB Post reply
	> Did you tried with dedicated VGA with built-in CSM without CSMWRAP? Without CSMWRAP freedos usb isn't detected at all in boot time. I don't have dedicated VGA with built-in CSM, I understand you mean an old Graphic card with VGA support.

RayeR

CZ,
07.09.2026, 19:39

@ FFK

Cannot boot DOS on ASRock B850M Pro with CMS from USB

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In my case the system recognize there is bootable legacy USB stick inserted and in boot menu it let me choose to boot legacy way or UEFI way (I have both there). Maybe if I could try to load simple MBR code that will e.g. ligt up capslock LED on the keyboard. But DOS disn'y booted because even ctrl+alt+del didn't work, it just hung...

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DOS gives me freedom to unlimited HW access.

samwdpckr 05.09.2026, 13:25 @ RayeR	Cannot boot DOS on ASRock B850M Pro with CMS from USB Post reply
	There are many new motherboards where the traditional BIOS API is completely dysfunctional, even though the option is there and it is advertised by the manufacturer. Have you tried to boot a more common operating system, such as some Linux-based system? Most likely that also fails.

Laaca

Czech republic,
05.09.2026, 18:06

@ samwdpckr

Cannot boot DOS on ASRock B850M Pro with CMS from USB

Post reply

> There are many new motherboards where the traditional BIOS API is
> completely dysfunctional, even though the option is there and it is
> advertised by the manufacturer. Have you tried to boot a more common
> operating system, such as some Linux-based system? Most likely that also
> fails.

Well, we need some opposite attitude - recomendation of motherboards (notebooks) where BIOS/CSM works or where works decently CSMwrap and where is possible to run DOS.

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DOS-u-akbar!

RayeR

CZ,
08.10.2026, 23:01

@ RayeR

Another legacy boot attempt on Gigabyte B850I Aorus Pro

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I have a chance to play with this MB, CPU AMD Ryzen 7 8600G, updated to the latest UEFI F13d

UEFI has built-in CSM option but itself is the same useless as with previous MB - it hangs with blinking cursor when trying to legacy boot from USB flash...

Then I tried with the latest CSMWrap 3.1.2
With IGPU it use fail-safe SeaVGA BIOS emulation that can't do much usefull.
So I pushed in GTX 730 with legacy VGA BIOS and CSMWrap can use it. In this case it's needed to have UEFI CSM option enabled otherwise CSMWrap hangs at splashscreen. With this legacy GPU I can run normal textmode DOS apps and also it passed VESATEST even enabling MTRR WC (cca 1800MB/s, 1920x1200/32bpp). It also can run Doom (playing demo) but keyboard control doesn't work - it only react to CTRL+ALT+DEL...

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DOS gives me freedom to unlimited HW access.

Zyzzle

09.10.2026, 09:40

@ RayeR

Another legacy boot attempt on Gigabyte B850I Aorus Pro

Post reply

> I have a chance to play with this MB, CPU AMD Ryzen 7 8600G, updated to the
> latest UEFI F13d
>
> UEFI has built-in CSM option but itself is the same useless as with
> previous MB - it hangs with blinking cursor when trying to legacy boot from
> USB flash...
>
> Then I tried with the latest CSMWrap 3.1.2
> With IGPU it use fail-safe SeaVGA BIOS emulation that can't do much
> usefull.
> So I pushed in GTX 730 with legacy VGA BIOS and CSMWrap can use it. In this
> case it's needed to have UEFI CSM option enabled otherwise CSMWrap hangs at
> splashscreen. With this legacy GPU I can run normal textmode DOS apps and
> also it passed VESATEST even enabling MTRR WC (cca 1800MB/s,
> 1920x1200/32bpp). It also can run Doom (playing demo) but keyboard control
> doesn't work - it only react to CTRL+ALT+DEL...
Good progress. I guess CSMWrap will never work with an Intel GPU BIOS. You needed to physically connect the GTX 730 in order to use its BIOS with CSM Wrap? This means CSMWrap won't work with modern UEFI-only laptops which of course don't have PCIE slots. VESA modes and LFB won't work or isn't present with the basic SeaBios VGA-only bios. Or did I misunderstand and you merely loaded the binary vBIOS of the GTX 730 card into CSMWrap and did not need to physically connect the card itself to the system?

Your MTRR Writecombining speed was impressive! For a UEFI-only system this video speed makes playing even demanding games possible with CSMWrap.

RayeR

CZ,
10.10.2026, 05:41

@ Zyzzle

Another legacy boot attempt on Gigabyte B850I Aorus Pro

Post reply

> Or did I misunderstand and
> you merely loaded the binary vBIOS of the GTX 730 card into CSMWrap and did
> not need to physically connect the card itself to the system?

Yes I needed to have GTX 730 physically in PCIe x16 slot. It would be possible to load vbios image from file with csmwrap but without presence od physical VGA the vbios would be useless. Ryzen 8xxxG simply don't have legacy vbios so no way there...
There's some issue with csmwrap kbd handling - it didn't correctly type a backslash \ and it didn't respond in doom that probably use it's own PM handler...

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DOS gives me freedom to unlimited HW access.