INT 15,3 - Write Blocks to Cassette (PC,PCjr only)

	AH = 03
	CX = count of bytes to write
	ES:BX = pointer to data buffer

	on return:

	CX = zero
	ES:BX = pointer to byte following last byte written

	- data is actually written in 256 byte blocks
	- if CX is less than 256, then the block is padded
	- no error detection/correction is available for writing