IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

BIOS Cassette Logic
Software Algorithms
Interrupt 15
The cassette routine will be called with the request type in AH and ~
the address of the bytes to be read or written will be specified by
(ES):(BX) and the number of bytes to read/write will be specified by
(CX). The actual number of bytes read will be returned in (DX).
Read block and write block will automatically tum the motor on at the
start and off at the end. The requests are as follows:
(AH) = 0 Tum the cassette motor on.
(AH) = 1
(AH) = 2
(AH) = 3
STATUS:
AH=OO
AH=Ol
AH=02
AH=04
AH=80
Note:
Cassette Write
Tum the cassette motor off.
(Read Block ) Read (CX) bytes into
memory beginning at address (ES):(BX)
and return actual number of bytes read in
(DX). Return the cassette status in (AH).
(Write Block) Write (CX) bytes onto the
cassette beginning at address (DS):(BX).
Return the cassette status in (AH).
No errors
CRC-Error (Read Block)
No data transitions
No leader
Invalid command
The carry flag will be set on any error.
The WRITE BLOCK routine writes a tape block on the cassette. The
tape block is described in Data Record Architecture page (3-10).
The WRITE BLOCK routine turns on the cassette motor and a syn­
chronization bit (0) and then writes 256 bytes of all ones, the leader,
to the tape. Next, one or more data blocks are written (depends on
number in CX). After each data block of 256 bytes, a two byte
CRC is written. The data bytes are taken from the memory location
pointed at by ES.
The WRITE BYTE routine disassembles the byte and writes it a
bit at a time to the cassette. The method used is to set TIMER 2 to
the period of the desired data bit. The timer is set to a period of
1.0 millisecond for a one bit and 0.5 millisecond for a zero bit.
3-8