Upgrade MSX2 to MSX2+ (Yamaha YIS-503III) КУВТ2

I think I can help you. I generate an Russin msx2+ rom for you. Begin of next week In will send you one. Does it have an F4 register?

Why would you do that? Do you want one?

switch, I do not understand

I am not an Electronic expert, but does it mean that bit 7 is 1 at cold startup?

to MSX Assembly Page
MSX I/O ports overview
This register is basically a simple memory cell which is preserved after a reset. It is used on the MSX2+ to differentiate between a hard boot (power on) and a soft boot (reset button) by writing a value to it during the boot routine, and check for that value everytime the computer starts. On the turboR, it also indicates whether the Z80 or the R800 has to boot.
Port range Description
#F4 System flags, preserved after reset
bit 5: CPU boot mode (1=R800)
bit 7: Boot method (1=soft boot, doesn’t show start logo)