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