The IAS Simulator

This document describes the simulator for the IAS computer.

The simulator may be run on any CIS Linux machine via:

~cs611/bin/iasim

The simulator will print a console prompt to stdout and will accept console commands from stdin.

The following commands may be used at the console:

• clr: Zero out all registers and memory.

• echo: Toggles an internal flag as to whether console commands are echoed before they are executed. Initially the flag is set to not echo. Enabling the echo is useful if the console commands are being provided via redirection from a file.

• help: Provide a quick summary of the console commands.

• in filename: Connect a file to the card reader. The file should contain a sequence of 40-bit words stored in Little Endian fashion. The machine's INCARD instruction will read 12 words per card. If a file was previously connected to the card reader, that file will be closed before the new file is opened.

• off: Power off the machine. The simulator will terminate.

• out filename: Connect a file to the card punch. If a file was previously connected to the card punch, that file will be closed before the new file is opened. The new file will be opened for writing. The machine's OUTCARD instruction will write 12 40-bit words per card. The words are written to the file in Little Endian format.

• run: Start the simulated machine running. The machine will start fetching instructions from where the PC points. Execution will continue until either an illegal instruction is encountered or the PC overflows. Note that there is no HALT instruction on the machine, but fetching 20 bits that contain all 0s will serve the purpose, since this is an illegal instruction and the machine will halt.

• set: Assign a value to a memory location or register. Here are some examples:
  • set ac 100
  • set mq 0x1a
  • set pc 2
  • set mem 100 19
  • set ac 0x00010 0x76ab3
  • set mem 101 0x000aa 0xa324e
  Note that the values can be specified in either decimal or hexadecimal. If the value is given in a single specification, then the value must fit in 32 bits. If a larger value is needed, then it should be specified as two 20-bit values: the high 20 bits first, followed by the low 20 bits.

• show: Display the contents of a memory location or a register. Here are some examples:
  • show ac
  • show mq
  • show pc
  • show mem 100
  • show mem 0x10c
  • show mem 100 10
  Contents are displayed using hexadecimal. Note that the memory address can be specified in either decimal or hexadecimal. When displaying memory contents, you can optionally specify a length: the number of words of memory you want to display.

• trace: Toggles an internal flag as to whether the simulator will print a trace while the machine is executing a program. Initially the trace flag is off. As each instruction executes, the trace displays the current PC value, disassembles the current instruction, displays the AC and displays the MQ.

For example, to run the factorial program (~cs611/public/prog2/fact.asm) you would first run the assembler and then use the following sequence of console commands:

set mem 0 0x78000
set mq 7
set pc 0
in fact.obj
run
set pc 2
in in.ias
out out.ias
run
off

The first command sets memory word 0 to hold an INCARD instruction in bits 0-19 and all zeroes in bits 20-39. This INCARD instruction has an address field of 0 and it will be used to read the object file for the factorial program into memory starting at location 0.

The INCARD instruction uses the contents of MQ to indicate how many cards to read. The assembler prints how many cards it produces when it translates an assembly language file into an object file. For the factorial program, seven cards are produced so we must set MQ to hold seven.

Next the PC is set to zero. This will cause the INCARD instruction to execute when the run command is subsequently used.

Then the in command is used to connect the object file for the factorial program to the simulator's card reader. The INCARD instruction will therefore read the object file for the factorial program.

Now, the run command is used. This causes the INCARD instruction to execute, which loads the factorial program's object file into memory. This will overwrite memory word 0 with the first word from the object file. This first word contains 0x0000000054 (84), which is the length of the object file. The machine will next try to execute bits 20-39 of the zero word, but these bits are all zero, so the machine will halt with an "illegal instruction" error.

At this point the factorial program is loaded into memory. Now the PC is set to 2, to skip over the object files two header words. Then the factorial input file and output file are connected to the card reader and the card punch. Then the factorial program can be run.

When the factorial program halts, then the off command is used to terminate the simulator.

With complicated load and execute sequences like this, of course, the easiest thing to do is to enter the console commands into a file (say, fact.run) and then run the simulator by redirecting the console commands from the file:
~cs611/bin/iasim <fact.run

Data files can be created for connecting to the IAS card reader by using the following program:
~cs611/bin/punchcards

The punchcards program reads ASCII decimal values from stdin and writes the corresponding IAS 40-bit words to stdout to represent IAS cards. An IAS card contains 12 words, so this program will pad the last card out with zeros. This program will only accept input values that will fit in 32 bits. The 40-bit values are written in Little Endian order to stdout.

Comments and questions should be directed to hatcher@unh.edu