IBM 5150 Technical Reference

BIOS Cassette Logic
Software Algorithms - Interrupt Hex 15
The cassette routine is called by the request type in AH. The
address of the bytes to be read from or written to the tape
is
specified ES:BX and the number of bytes to be read or written is
specified by ex. The number of bytes read is returned in DX.
The read block and write block automatically turn the cassette
motor
on at the start and off at the end. The request types in AH
and the cassette status descriptions follow:
Request Type Function
AH
= 0 Turn Cassette Motor On
AH
= 1 Turn Cassette Motor Off
AH = 2 Read Tape Block
Read CX
bytes into memory starting at Address ES:BX
Return actual
number of bytes read in DX
Return Cassette
Status in AH
AH = 3
Write Tape Block
Write CX bytes onto cassette starting at Address DS:BX
Return Cassette
Status in AH
Cassette Status Description
AH
= 00 No Errors
AH
= 01 Cyclic Redundancy Check (CRC) Error in Read Block
AH
= 02 No Data Transitions
AH
= 04 No Leader
AH
= 80 Invalid Command
Notes: The carry flag will be
set on any error.
Cassette Write
The write-block routine writes a tape block onto the cassette tape.
The tape block
is described in "Data Record Architecture" later
in this section.
The write-block routine turns
on the cassette drive motor and a
synchronization bit(O) and then writes the leader(256 bytes of all
l's) to the tape. Next, the routine writes the number of data
System BIOS 5-25

IBM 5150 Specifications

General

Processor	Intel 8088
Clock Speed	4.77 MHz
RAM	16 KB - 256 KB
Manufacturer	IBM
Model	5150
Release Year	1981
Storage	5.25-inch floppy drives
Graphics	MDA or CGA
Operating System	IBM PC DOS
Ports	Cassette, keyboard
Expansion Slots	5