IBM 5150 Hardware Reference Manual

The timer is set mode 3 which means it will output a square wave
with period given by its count register. The timer's period is changed on
the
fly for each data bit to be written to the cassette. If the number of
data bytes to be written is not an integral multiple off 25 6, then after the
last desired data byte from memory has been written, the data block will
~ be extended to 256 bytes by writing multiples of the last data byte. The
last block will be closed with two
CRC bytes as usual. After the last
data block, a trailer consisting
of four bytes of all one bits will be
written. Finally, the motor will be turned
off. There are no errors
reported by this routine.
\.-- 250 USEC ----.j
I
ZERO BIT
500 USEe
r
·1
lONE BIT
~
1000 USEC
I~
·1
Cassette Read
The READ BLOCK routine turns on the cassette motor and then
delays for approximately 0.5 secs for it
to come up to speed.
The
READ BLOCK routine then searches for leader and must
detect
all one bits for approximately 1/4 ofleader length before it
can look for the sync byte.
If a correct sync byte (X '16') is not
found, the routine goes back and searches for leader again.
The data is read a bit a a time and assembled into bytes. After
each byte
is assembled it is written into memory at location
ES:BX and then BX is incremented by one.
After each multiple
of 256 data bytes are read, the CRe is read
and compared to the
eRe generated. If a eRe error is detected,
the routine will exit with the
carry flag set to indicate an error
and status (AH) - 01 for
eRe error. DX will contain the number
of bytes written into memory.
Note: The Time
of Day Interrupt (IRQO) is disabled during the
cassette read operation.
3-9

IBM 5150 Specifications

General

Brand	IBM
Model	5150
Category	Desktop
Language	English