pc.4

UNIX v6源代码 这几乎是最经典的unix版本 unix操作系统设计和莱昂氏unix源代码分析都是用的该版

.th PC IV 10/15/73
.sh NAME
pc  \*-  PC-11 paper tape reader/punch
.sh DESCRIPTION
.it Ppt
refers to the PC-11 paper tape reader or punch,
depending on whether it is read or written.
.s3
When
.it ppt
is opened for writing, a 100-character leader
is punched.
Thereafter each byte written is punched on
the tape.  No editing of the characters
is performed.
When the file is closed, a 100-character trailer is punched.
.s3
When
.it ppt
is opened for reading, the process
waits until tape is placed in the reader and the reader
is on-line.
Then requests to read cause the characters read
to be passed back to the program, again without
any editing.
This means that several null leader characters
will usually appear at the beginning of the file.
Likewise several nulls are likely to appear at the end.
End-of-file is generated when the tape runs out.
.s3
Seek calls for this file are meaningless.
.sh FILES
/dev/ppt
.sh BUGS
If both the reader and the punch are open simultaneously,
the trailer is sometimes not punched.
Sometimes the reader goes into a dead state in which it cannot be
opened.