Support IOC commands for disk/drum devices

Thanks to Kevin Brunelle

There is a minor fix included with regards to tape devices. The test
was failing if M == 0, when it should fail when M != 0.

NOTICE: This patch changes the behavior of the VM and changes the
function parameters for the ioc_ function. Documentation changes are
included.

Permits the following:
     LDX   BLKNUM
     IOC   0(8)
     OUT   ADDR(8)      Write block from ADDR into disk[BLKNUM]
     IOC   0(8)
     IN    ADDR(8)      Read block from disk[BLKNUM] into ADDR
...
BLKNUM  CON  45000      Example possible block on disk

I was having an issue writing a block to a drive and then reading back
the same block. Because it is impossible to move the SEEK_CUR pointer
backwards on a disk device, there was no way for a program to read
back a block that it wrote to a disk without restarting or fiddling
with ~/.mdk/disk?.dev files and symbolic links.

I have added a function parameter to the ioc_ function and used it to
pass the value of rX to ioc_. This permits us to use IOC commands to
move the read/write head on a disk/drum device. I believe that this
conforms to the potential meaning of Knuth's description of IOC for
disk/drum devices.

I have put in tests to verify that rX is positive and M = 0.

I have updated the documentation to reflect this new behavior.

This makes disks much more usable.

Note: I won't be offended if this patch is rejected because it changed
the behavior of the VM. I think it fits the spirit and enhances the
functionality in a way that some might find useful. I wanted it for
something I was working on, and I felt others might want the same. The
thing with the paper-tape should be fixed, though.

1 files changed, 11 insertions, 8 deletions

diff --git a/doc/mdk_tut.texi b/doc/mdk_tut.texi
index e2d3a15..9ff6f86 100644
--- a/doc/mdk_tut.texi
+++ b/doc/mdk_tut.texi
@@ -684,14 +684,17 @@ OPCODE = 34, MOD = I/O unit.
 @noindent
 In all the above instructions, the @samp{MOD} subfile must be in the
 range 0-20, since it denotes the operation's target device. The
-@samp{IOC} instruction only makes sense for tape devices (@samp{MOD} =
-0-7 or 20): it shifts the read/write pointer by the number of words
-given by @samp{M} (if it equals zero, the tape is rewound)@footnote{In
-Knuth's original definition, there are other control operations
-available, but they do not make sense when implementing the block
-devices as disk files (as we do in @sc{mdk} simulator). For the same
-reason, @sc{mdk} devices are always ready, since all input-output
-operations are performed using synchronous system calls.}.
+@samp{IOC} instruction makes sense for magnetic tape devices (@samp{MOD} =
+0-7): it shifts the read/write pointer by the number of blocks
+given by @samp{M} (if it equals zero, the tape is rewound), paper tape
+devices (@samp{MOD} = 20): @samp{M} should be 0, the tape is rewound,
+and disk/drum devices (@samp{MOD} = 8-15): it moves the read/write
+pointer to the block specified in rX and @samp{M} should be 0@footnote{In
+Knuth's original definition, there are other control operations available,
+but they do not make sense when implementing the devices as disk files (as
+we do in @sc{mdk} simulator). For the same reason, @sc{mdk} devices are
+always ready, since all input-output operations are performed using
+synchronous system calls.}.
 
 
 @node Conversion operators, Shift operators, Input-output operators, MIX instruction set