Stage 3 : Bootstrap Loader (2 Hours)

Learning Objectives

  • Use the XSM Instruction set to write a small OS startup code.
  • Load your OS startup code into the boot block of the disk and get this code executed on bootstrap.

Pre-requisite Reading

When the XSM machine is started up, the ROM Code, which resides in page 0 of the memory, is executed. It is hard-coded into the machine. That is, the ROM code at physical address 0 (to 511) is "already there" when machine starts up. The ROM code is called the "Boot ROM" in OS literature. Boot ROM code does the following operations :

  1. Loads block 0 of the disk to page 1 of the memory (physical address 512).
  2. After loading the block to memory, it sets the value of the register IP(Instruction Pointer) to 512 so that the next instruction is fetched from location 512 (page 1 in memory starts from location 512).

In this stage, you will write a small assembly program to print "HELLO_WORLD" using XSM Instruction set and load it into block 0 of the disk using XFS-Interface as the OS Startup Code. As described above, this OS Startup Code is loaded from disk block 0 to memory page 1 by the ROM Code on machine startup and is then executed.

The steps to do this are explained in detail below.

1) Create the assembly program to print "HELLO_WORLD". The assembly code to print "HELLO_WORLD" : 

MOV R0, "HELLO_WORLD"
MOV R16, R0
PORT P1, R16
OUT
HALT
Save this file as $HOME/myexpos/spl/spl_progs/helloworld.xsm.

2) Load the file as OS Startup code to disk.xfsusing XFS-Interface.
Invoke the XFS interface and use the following command to load the OS Startup Code 

cd $HOME/myexpos/xfs-interface
./xfs-interface
# load --os $HOME/myexpos/spl/spl_progs/helloworld.xsm
# exit

Note that the --os option loads the file to Block 0 of the XFS disk.

3) Run the machine

cd $HOME/myexpos/xsm
./xsm

The machine will halt after printing "HELLO_WORLD".

HELLO_WORLD
Machine is halting.

Note

The XSM simulator given to you is an assembly language interpeter for XSM. Hence, it is possible to load and run assembly language programs on the simulator (unlike real systems where binary programs need to be supplied).

If the OS Startup Code is loaded to some other page other than Page 1, will XSM work fine?

No. This is because after the execution of the ROM Code, IP points to 512 which is the 1st instruction of Page 1. So if the OS Startup Code is not loaded to Page 1, it results in an exception and leads to system crash.

Assignment 1

Write an assembly program to print numbers from 1 to 20 and run it as the OS Startup code.

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