C:\>flashrom.exe -p internal -r backup.bin
[ this worked flawlessly ]
apply mods with UBU to this bin, rename to MOD.bin

C:\>flashrom.exe -p internal -w MOD.bin
Flashrom on FreeDOS 7 (i786)
flashrom is free software, get the source code at https://flashrom.org

Calibrating delay loop... delay loop is unreliable, trying to continue OK.
Found chipset "AMD FP4/FP5/AM4".
Enabling Flash write... OK.
Found Macronix flash chip "MX25U25635F" (32768kB, SPI) mapped at physical address 0xfe000000.
Error: Failed to read complete file. got 30621696 bytes, wanted 33554432!