@Dietmar @infuscomus
 
	Yeeeeeeeaaaaaaaaaaa!!! 😀😁😛😜
 
	It finally worked ! Tested debug WinXP SP2 64-bit
 
	
		If the PC has a integrated COM port on the motherboard, turn it off in the bios
	
	
		I plugged my PCIe RS232 Sunix SER5427A card into the PCIe x1 slot
	
	
		Check I/O port my card in Debian or Windows with pciutlis - lspci -s xx:xx.x -v, my card has D000
	
	
		Mod kdcom.dll x64 in offset 1024 change F803 to 00D0 (little endian)
	
	
		Replace kdcom.dll in WINDOWS\system32 on debuged PC
	
	
		We set up debug in boot.ini:
		/debug /debugport=COM1: /baudrate=14400
	
	
		On host PC run WinDbg baudrate 115200
	
	
		Boot debugged PC to MS-DOS 6.22 with tools debug.exe and grub.exe (GRUB4DOS)
	
	
		Insert I/O addres to memory 40:0 for COM1 (my is D000) (or simply run sundos.exe) 
		debug 
		-e 40:0 00 D0 
		-q
	
	
		Run GRUB4DOS (grub.exe)
	
	
		Boot WinXP (menu.lst):
		default 0
timeout 5
title Boot WinXP
map (hd0) (hd1)
map (hd1) (hd0)
map --hook
find --set-root /ntldr
chainloader /ntldr
	
	P.S. If on a computer with WinDbg we have a modern PCIe card with an RS232 port that allows higher speeds, then you can set the maximum to what both PCIe cards allow, for me you can do this:
 
	boot.ini - 57600 & WinDbg 460800😁