273. ENHANCEMENT: Burst-fill only the first of two MPX receive buffers in FASTTIME mode.
OBSERVATION: When the 8-channel multiplexer is set for "optimized timing"
mode, buffered characters are transferred in blocks to and from the Telnet
connection. That is, the line service routine will send or receive
characters as long as they are available. This is more efficient than the
"realistic timing" mode, which sends or receives one character per service
invocation. Effectively, this means that up to 508 characters (two buffers
of 254 bytes each) may be sent or received between one CPU instruction or
DCPC cycle and the next. This works well for sending, but it can cause
buffer overflows when receiving.
Consider an application (such as Kermit) that receives large blocks of data
at high speed from a client. The multiplexer is designed to handle this
condition by interrupting the CPU when the first buffer is filled and
filling the second buffer while the CPU is unloading data from the first.
In realistic mode at 19,200 baud, the CPU has approximately 800
instructions or DCPC cycles available per character received. With a
second buffer of 254 bytes, the CPU has approximately 203,000 instructions
available to unload the first buffer after receiving the interrupt
notification. Once started, the DCPC transfer takes no more than 508
instruction times, so the CPU can easily keep up with data arriving at the
maximum baud rate.
In fast timing mode, however, the first buffer burst-fills in a single CPU
instruction time, and, if available from the Telnet connection, the second
buffer fills in the next instruction time. At that point, any additional
characters received will result in a buffer overflow condition. The
problem is that the CPU has no time between the first burst and the second
to empty the first buffer.
RESOLUTION: Modify "mpx_line_svc" (hp2100_mpx.c) to shift from burst
transfers to character-at-a-time transfers when a receive buffer is full
and awaiting unloading by the CPU. This allows the CPU and DCPC time to
read the buffer contents into memory before the second multiplexer buffer
is full. Once the completed buffer is freed, the service routine returns
to burst mode to fill the remainder of the other buffer, permitting the
efficiency of block transfers while avoiding buffer overruns with large
data transfers.
274. PROBLEM: A second connection to the BACI device leaves the client unresponsive.
OBSERVATION: The BACI device supports a single terminal client connection.
If a second connection is attempted, the client connects but is otherwise
unresponsive. It would be better if the client received the "All
connections busy" message that is reported by the terminal multiplexers
(MPX and MUX devices) when the number of connections is exceeded.
CAUSE: The "baci_poll_svc" is calling the "tmxr_poll_conn" routine only if
the port is not connected. The routine should be called unilaterally, so
that it will report an error and disconnect the client when all lines are
in use and another connection is attempted.
RESOLUTION: Modify "baci_poll_svc" (hp2100_baci.c) to call
"tmxr_poll_conn" unconditionally, so that a second concurrent connection
attempt will be rejected with "All connections busy".
275. PROBLEM: The exported program counter name (PC) clashes with other libraries.
OBSERVATION: In HP 21xx/1000 systems, the P register is the program
counter. In keeping with the naming of the other register variables (e.g.,
for A register, B register, etc.) in the simulator, the variable used
should be named "PR". However, for traditional reasons, the program
counter in SIMH is named "PC".
The main CPU module declares its hardware register variables as global, so
that they may be accessed by other CPU helper modules. Unfortunately, the
"curses" library also declares the symbol "PC" as global, leading to
conflicts when it is loaded by SCP. A workaround had been implemented that
renamed "PC" to "PC_Global" in the HP2100 simulator, but that meant that
the new name had to be used when debugging, which was awkward.
CAUSE: A poor choice of global symbol names from the "termcap" library,
which was inherited by the "curses" library.
RESOLUTION: Change the program counter variable name from "PC" to "PR"
(hp2100_cpu.c, hp2100_cpu1.c, hp2100_cpu2.c, hp2100_cpu3.c, hp2100_cpu4.c,
hp2100_cpu5.c, hp2100_cpu6.c, hp2100_cpu7.c, hp2100_dr.c, and hp2100_ipl.c)
to avoid a name clash and for register naming consistency.
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