Cassette Read - IBM 5150 Technical Reference

blocks specified by
ex.
After each data block of 256 bytes, a
2-byte cyclic redundancy check (eRe) is written. The data bytes
are taken from the memory location pointed at by ES.
The write-byte routine disassembles and writes the byte 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-ms for a I bit and 0.5-ms for a 0 bit.
The timer is set to mode 3, which means the timer outputs a
square wave with a period given by its count register. The timer's
period is changed on the fly for each data bit written to the
cassette.
If
the number of data bytes to be written is not an
integral multiple of 256, then, after the last desired data byte from
memory has been written, the data block is extended to 256 bytes
of writing multiples of the last data byte. The last block is closed
with two eRe bytes as usual. After the last data block, a trailer
consisting of four bytes of alII bits is written. Finally, the
cassette motor is turned off, if there are no errors reported by the
routine.
1+--250 fJS
~
Zero Bit
1~4--- 500 fJS ---~.I
'----_----'I .
- One Bit
~14---------------1000fJs--------------~·1
Cassette Read
The read-block routine turns on the cassette drive motor and then
delays for about 0.5 second to allow the motor to come up to
speed.
The read-block routine then searches for the leader and must
detect alII bits for approximately 1/4 of the leader length before
it can look for the sync (0) bit. After the sync bit is detected, the
5-26
System
BIOS