Neat hack: by smuggling SeaBIOS into a PE/COFF wrapper you get a soft-CSM without touching flash. One gotcha I have seen on some late-gen Intel boards is that SMM is locked until after ExitBootServices, so real-mode INT 15 tricks (think A20 dance) that SeaBIOS relies on can fail silently and leave DOS hanging. If that bites you, test in OVMF first or look for a firmware option that relaxes SMM lockdown.
Hit the same wall on a Coffee Lake NUC; flipping the UEFI third-party Option ROMs toggle surprisingly re-enabled the SMI handler early enough for SeaBIOS to clear A20, so always worth spelunking those hidden menus before blaming CSMWrap.
Have also had success by rebuilding SeaBIOS with CONFIG_LATE_A20=1, which defers the A20 gate dance until after ExitBootServices so the locked SMM handler is no longer required. It is ugly, but on Alder Lake laptops it let DOS/Win98 boot without poking at vendor menus.
Neat hack: by smuggling SeaBIOS into a PE/COFF wrapper you get a soft-CSM without touching flash. One gotcha I have seen on some late-gen Intel boards is that SMM is locked until after ExitBootServices, so real-mode INT 15 tricks (think A20 dance) that SeaBIOS relies on can fail silently and leave DOS hanging. If that bites you, test in OVMF first or look for a firmware option that relaxes SMM lockdown.
Hit the same wall on a Coffee Lake NUC; flipping the UEFI third-party Option ROMs toggle surprisingly re-enabled the SMI handler early enough for SeaBIOS to clear A20, so always worth spelunking those hidden menus before blaming CSMWrap.
Have also had success by rebuilding SeaBIOS with
CONFIG_LATE_A20=1, which defers the A20 gate dance until after ExitBootServices so the locked SMM handler is no longer required. It is ugly, but on Alder Lake laptops it let DOS/Win98 boot without poking at vendor menus.