|Int 19H is the last action performed by the BIOS before handing control over to the Operating System.|
INt19H is a software interrupt that putss address 0019H onto the address bus and starts executing code found there. The code will have been put there by the BIOS. This code usually is just a jump instruction which jumps to a place in the BIOS where the code is.
Int19H loads the data found a Head 0, Track 0, Sector 1 which is at the very beginning of the disk. On a hard disk this will be the Master Boot Record. This loads the bit of code found there and executes it which then proceeds to load the lest of the Operating System. If Int19 executes correctly but cannot find find a hard or floppy disk to load from it comes beack with No Operating system Error.
As you are not getting such an error it suggests to me that Int 19 is not getting executed which means it is either a motherboard failure or a memory failure.