This grub4dos helper utility allows you to make any BIOS call you like. You can setup the registers and which interrupt you want to use, it then makes the BIOS call and prints out the CPU register contents after the call returns, like this:
EAX=00008600 EBX=00000000 ECX=00000000 EDX=00000000 ESI=00000000 EDI=00000000 EBP=00000000 ESP=00006284 EIP=00000000 eFLAG=00000A83 DS=0000 ES=0000 FS=0000 GS=0000 SS=0000 CS=0000 OV UP EI NG NZ NA PO CY
The flag registers are displayed so you can test for the Carry Flag (NC or CY) or Zero flag (NZ or ZR).
This allows you to make any BIOS call (for instance write characters to a certain position on the screen, read a disk sector, etc.).
You can then check these registers by redirecting the output to memory and using cat - e.g. a single line in a grub4dos menu ...
# if plop is not installed, then run it to reboot from USB in USB 2.0 mode /plpbt/bios int=0x13 eax=0x504c6f50 ebp=0x43484b44 > (md)0x300+1 && cat --locate="EAX=79657320" (md)0x300+1 || pause --wait=3 loading Plop! && kernel /plpbt/plpbt.bin hiddenusb && boot
Do not experiment with int13h calls unless you are careful and know what you are doing as they can be dangerous (e.g. could trash your hard disk!).