Added page describing use of SIMH to simulate a PDP-11 for running code compiled...

[website_subgeniuskitty.com] / data / development / pdp-11 / modern_c_software_development / pdp11-simulator-simh.md

# Overview #

TODO: Write introduction. Goal is to run baremetal PDP-11 programs in SIMH, a
PDP-11 simulator.

TODO: What kind of joint header do I want across all the articles in a set,
linking them together?

# SIMH Installation #

We first need to install [SIMH](https://github.com/simh/simh), a program that
simulates many computer architectures including the DEC PDP-11.

  - **FreeBSD:** Either install package via `pkg install simh` or build from
    ports under `emulators/simh`.

  - **Debian:** Install as package via `sudo apt install simh`.

  - **Windows:** Download [pre-built snapshot binaries](https://github.com/simh/Win32-Development-Binaries).

  - **Other:** Read the [SIMH README](https://github.com/simh/simh) section
    "Building Simulators Yourself".

After installation, launch the PDP-11 simulator with the command `pdp11`. It
should display a prompt `sim>` where you can type `quit` followed by `ENTER` to
exit the simulator.

    % pdp11
    
    PDP-11 simulator V3.9-0
    sim> quit
    Goodbye
    %


# SIMH Basics #

Now that SIMH is installed, go ahead and launch it again with the command
`pdp11`. This should result in the prompt `sim>`. All commands entered at this
`sim>` prompt are executed by the SIMH simulator itself, not by the simulated
PDP-11. For example, we could display the configuration of the simulated
PDP-11 defined in SIMH.

    sim> show configuration
    PDP-11 simulator configuration

    CPU, 11/73, NOCIS, idle disabled, autoconfiguration enabled, 256KB SYSTEM
    [...]

We can change the simulated PDP-11, for example by changing to a different
model.

    sim> set cpu 11/40
    Disabling XQ
    sim> show configuration
    PDP-11 simulator configuration
    
    CPU, 11/40, NOFIS, idle disabled, autoconfiguration enabled, 256KB SYSTEM
    [...]

SIMH also allows us to deposit values directly into memory and read them back.
Note that addresses in SIMH refer to the physical address rather than the
virtual address, and thus may be up to 22-bits long on certain models, as shown
below.

    sim> examine 0140000
    140000: 000000
    sim> deposit 0140000 042
    sim> examine 0140000
    140000: 000042
    sim>

Once a program is loaded into memory, the simulated PDP-11 may be commanded to
execute it via the command `go <address>`. For example, if the program was
loaded starting at address `01000`, then begin execution with `go 01000`.


## Interrupting Simulation ##

At any time, the simulation may be paused by pressing `Ctrl-e`. This returns to
the `sim>` prompt where memory may be examined or other extra-sim capabilities
executed. For example, if we have the PDP-11 CPU execute a tight infinite loop,
the simulation never naturally ends, but we can pause it, allowing us to exit.

    sim> go
    <Press Ctrl-e>
    Simulation stopped, PC: 004166 (BR 4166)
    sim> quit
    Goodbye
    %

Note that, instead of `quit`, typing `go` would have resumed the simulation
exactly where it paused, shown in the status message where the `PC` register is
set to address `04166`.


## Saving Configuration ##

Any command that may be entered at the `sim>` prompt, may also be entered in a
configuration file. For example, consider the following file named `simh.conf`.

    deposit 01000 0777
    echo Just set address 01000 to the instruction 'BRANCH 01000'.
    go 01000

We can tell SIMH to load this file and execute each line as though it were
typed at the `sim>` prompt by including the config file name. Even though the
commands are not displayed, we can see from the `echo` command that they were
executed.

    % pdp11 simh.conf
    
    PDP-11 simulator V3.9-0
    Just set address 01000 to the instruction 'BRANCH 01000'.
    
    Simulation stopped, PC: 001000 (BR 1000)
    sim> quit
    Goodbye

Since this program is an infinite loop, we pressed `Ctrl-e` to pause the
simulation and allow us to quit.


# SIMH Loader #

Since we're trying to run bare-metal code on this simulated PDP-11, we don't
need to bother with disk images; instead we will load a binary directly into
the PDP-11's memory using SIMH's `load` command.


## Loader Format ##

The loader included with SIMH doesn't accept a raw binary file or the a.out
executable generated by our cross compiler. Instead, it expects a paper tape
image file in the following format.


    Loader format consists of blocks, optionally preceded, separated, and
    followed by zeroes. Each block consists of the following entries. Note
    that all entries are one byte.
    
    0001
    0000
    Low byte of block length (data byte count + 6 for header, excludes checksum)
    High byte of block length
    Low byte of load address
    High byte of load address
    Data byte 0
    ...
    Data byte N
    Checksum
    
    The 8-bit checksum for a block is the twos-complement of the lower eight
    sum bits for all six header bytes and all data bytes.
    
    If the block length is exactly six bytes (i.e. only header, no data),
    then the block marks the end-of-tape. The checksum should be zero.  If
    the load address of this final block is not 000001, then it is used as
    the starting PC.

If you don't want to generate this format yourself, use the utility
[bin2load](https://git.subgeniuskitty.com/pdp11-bin2load/.git) which converts a
binary image into a SIMH compatible loader format. See the `README.md` file for
installation instructions.


## Load and Execute ##

We can extract a binary from the a.out file generated by our cross compiler
using the `pdp11-aout-objcopy` tool built at the same time as the cross
compiler. This binary can then be converted by `bin2load` and loaded into SIMH.

    % pdp11-aout-objcopy --only-section=.text --output-target binary program.out program.bin
    % bin2load -i program.bin -o program.pdp11 -a 01000
    % pdp11

    PDP-11 simulator V3.9-0
    sim> load program.pdp11
    sim> go

If we pass a starting address to `bin2load` with the `-a` flag (as shown
above), then SIMH configures the simulation so that execution starts at address
`01000` when the `go` command is entered.

The `load <file>` and `go` command may also be included in the SIMH
configuration file, automatically loading and executing whenever the simulator
is started.


# Execution #

Let's bring all these steps together. Assume we've built our cross compiler and
created a `Hello, World!` program like the one shown in the four files below.

**`program.c`:**

    #include <stdint.h>
    
    #define XCSR (*((volatile uint16_t *)0177564))
    #define XBUF (*((volatile uint16_t *)0177566))
    
    void
    putch(uint16_t c)
    {
        while((XCSR && 0200) == 0) continue;
        XBUF = c;
    }
    
    void
    print_string(const char * string)
    {
        while (*string != '\0') {
            putch(*string);
            string++;
        }
    }
    
    void
    cstart(void)
    {
            volatile uint16_t * test_word = (volatile uint16_t *) 0140000;
            *test_word = 0123456;
    
            print_string("Hello, World!\r\n");
    }

**`bootstrap.s`:**

    .globl _start
    
    _start:
        mov $01000,sp
        jsr pc,_cstart
        halt

**`pdp11.ld`:**

    OUTPUT_FORMAT("a.out-pdp11")
    ENTRY(start)
    phys = 00001000;
    SECTIONS
    {
      .text phys : AT(phys) {
        code = .;
        *(.text)
        *(.rodata)
        . = ALIGN(0100);
      }
      .data : AT(phys + (data - code))
      {
        data = .;
        *(.data)
        . = ALIGN(0100);
      }
      .bss : AT(phys + (bss - code))
      {
        bss = .;
        *(.bss)
        . = ALIGN(0100);
      }
      end = .;
    }

**`simh.conf`:**

    load program.pdp11
    go 1000

Note that the program writes the value `0123456` to address `0140000` and then
prints the string `Hello, World!` to the console SLU before halting.

We can compile and execute this program with the following sequence of
commands. Note that after the program halts, we are able to examine address
`0140000` and see the value `0123456` written by the program.

    % pdp11-aout-as -o bootstrap.o bootstrap.s
    % pdp11-aout-gcc -c -Wall -Wno-unused-function -O0 -ffreestanding \
        -fomit-frame-pointer -fno-builtin-alloca -std=c99 -o program.o program.c
    % pdp11-aout-ld -T pdp11.ld --entry _start bootstrap.o program.o -o program.out
    % pdp11-aout-objcopy --only-section=.text --output-target binary program.out program.bin
    % bin2load -i program.bin -o program.pdp11 -a 01000
    % pdp11 simh.conf
    Paper tape will load at address 01000.
    
    PDP-11 simulator V3.9-0
    Hello, World!
    HALT instruction, PC: 001012 (BR 1016)
    sim> examine 0140000
    140000: 123456
    sim> quit
    Goodbye

TADA! Now you can test your programs on the simulator as you write them.


# Beyond Basics #

SIMH includes many features beyond the basics shown in this document. The
curious user may enter `help` at the `sim>` prompt for more information.

However, before leaving the topic of testing code on simulated PDP-11s, I want
to mention two simulated pieces of hardware provided by SIMH that can be of use
in this task.


## Line Printer ##

The DEC LP11 line printer is emulated by SIMH and its output is saved to a text
file during the simulation. To save output as `line_printer.txt`, execute the
following command in SIMH.

    sim> attach lpt line_printer.txt

The DEC LP11 is controlled by two registers, the Line Printer Status register
(`LPS`) located at `0177514` and the Line Printer Data Buffer register (`LPDB`)
at `0177516`. To use the LP11, simply test bit 7 of `LPS` for printer readiness
and then write a 7-bit character into the low bits of `LPDB`.

For example, you could use something like the following C code to print from
the simulation to the printer text file.

    #define LPS  (*((volatile uint16_t *)0177514))
    #define LPDB (*((volatile uint16_t *)0177516))
    
    void
    putch(uint16_t c)
    {
        /* Test bit 7 (aka: 0200) of LPS for readiness. */
        while((LPS && 0200) == 0) continue;
        /* Transfer a byte when ready. */
        LPDB = c;
    }

This provides an easy method for your PDP-11 program to output data to the host
which is automatically saved, all while requiring minimal code be added to the
PDP-11's program.


## Serial via Telnet ##

The DEC DC11 asynchronous line interface is used between the PDP-11 and a
serial asynch line. Via SIMH, these lines can be redirected to TCP ports which
are accessible via `telnet`. For example, to enable eight separate lines in
SIMH and listen on port `1170`, type the following at the `sim>` prompt or add
to your SIMH configuration file.

    sim> set dci en
    sim> set dci lines=8
    sim> set dci 1170

Now you can `telnet` to port `1170` and SIMH will redirect your connection to
the first available line of the simulated DC11.

Inside the simulation, interacting with a DC11 is fairly simple.

Four registers are associated with each serial line. The first is at addresses
`0177400`-`0177406`, the next at `0177410`-`0177416`, etc. Within a block of
four words, the registers are assigned as follows.

  - `1774xx0`: Receiver Status (`RCSR`)
  - `1774xx2`: Receiver Buffer (`RBUF`)
  - `1774xx4`: Transmitter Status (`XCSR`)
  - `1774xx6`: Transmitter Buffer (`XBUF`)

Your program should test bit 7 of the `RCSR` and `XCSR` registers to determine
when the serial line has received a byte and is ready for it to be read, or the
serial line is ready to transmit a byte. When ready, transfer a byte to the
lower half of `XBUF` or from the lower half of `RBUF`.

The code for these operations should look just like the code given above for the
LP11 line printer.

For a full description of the programming interface of the DC11, see the 
[PDP-11 Peripherals Handbook](http://www.bitsavers.org/pdf/dec/pdp11/handbooks/PDP11_PeripheralsHbk_1972.pdf)
starting on page 109.

Using these simulated serial lines, you can easily interact with your PDP-11
program via TCP ports, either manually using a `telnet` program, or
programmatically. This can be a powerful debugging tool.