1. SICvm A SIC Based Virtual Machine
Prepared by
Anoop U.Thomas
V Hariharan
Hari Sankar S
Santhosh N.Raju

2. SICvm – A SIC Based Virtual Machine
Introduction
In this project we aim to develop a Virtual Machine
The Machine will be based on the Hypothetical Machine described by Leland L.Beck in System Software called Simplified Instructional Computer (SIC)
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

3. A Background on Virtual Machines
The term virtual machine is used to refer to the environment created by an emulator, where software is used to emulate an operating system for the end user.
Church–Turing thesis - implies that any operating environment can be emulated within any other.
In practice, it can be quite difficult, particularly when the exact behavior of the system to be emulated is not documented and has to be deduced through reverse engineering
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

4. How to Achieve Virtualization ?
This can be done using three major ways:
Full Virtualization — the virtual machine simulates the complete hardware, allowing an unmodified OS for a completely different CPU to be run.
Paravirtualization — the virtual machine does not simulate hardware but instead offers a special API that requires OS modifications.
Native Virtualization — the virtual machine only partially simulates enough hardware to allow an unmodified OS to be run in isolation, but the guest OS must be designed for the same type of CPU.
In order to achieve Virtualization we need to have two things.
A Virtual Machine (VM)
A Virtual Machine Monitor (VMM)
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

5. SICvm – A SIC Based Virtual Machine
A Background On SIC
Simplified Instructional Computer (SIC) is a hypothetical computer described in Leland Beck's book System Software.
Most real microprocessors have lots of quirks built-in to increase efficiency.
SIC provides a much simplified view of systems hardware from the perspective of systems programmer
SIC has two versions, a standard version, SIC, and an extended version, SIC/XE.
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

6. SICvm – A SIC Based Virtual Machine
The SIC Architecture
Memory:
Word size: 3 bytes (24 bits)
Total size: 32,768 bytes (215). Thus any memory address will need at most 15 bits to be referenced ('almost' four hex characters)
Registers: Total Registers: Five
Accumulator (A): Used for most of the operations (number 0)
Index (X): Used for indexed addressing (number 1)
Linkage (L): Stores return addresses for JSUB (number 2)
Program Counter (PC): Address for next instruction (number 8)
Status Word (SW): Information and condition codes (number 9).
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

7. The SIC Architecture (Continued…)
Instruction Formats:
Opcode: First 8 bits, direct translation from the Operation Code Table
Flag (x): next bit indicates address mode (0 direct - 1 indexed)
Address: next 15 bits, indicate address of operand according to address mode.
Addressing Modes:
Only two possible addressing modes
Direct (x = 0): operand address goes as it is
Indexed (x = 1): value to be added to the value stored at the register x to obtain real address of the operand.
OPCODE X
ADDRESS
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

8. Why Choose SIC To Make A Virtual Machine ?
SIC is a generic view point of a system, that is simple to develop and implement compared to architectures like x86 etc
The SIC architecture is pretty flexible and extendible to other architectures just by simple modifications. This can be achieved in the VM by simple code modifications
SIC machine is used as a study tool for systems programming and this VM helps the better understanding of this machine
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

9. SICvm – A SIC Based Virtual Machine
The Planning Process
Virtual Machines can be designed in two ways
High Level Emulation (Top Down Approach)
Low Level Emulation (Bottom Up Approach)
We choose Low Level Emulation
The system to be developed was not a very complex system
The operational details of the system was known in detail
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

10. Issues Faced In The Planning Process
We couldn’t get enough detailed documentation on how to develop a Virtual Machine
Existing projects like Bochs and Xen were too complex and large to be studied in the given short period of time
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

11. The Development Process
The Development Process is divided into 3 Stages
Development of SICvm – The Virtual Machine
Development of the SIC Assembler
Development of the Essential System Programs for SIC
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

12. Tools Used in Development of SICvm
gcc – The GNU compiler collection
make utility
gdb – The GNU Debugger
emacs, vi etc – Text Editing
Windows / Linux Operating System Environment was used in development
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

13. SICvm – A SIC Based Virtual Machine
The SICvm Machine
Usually an emulator is divided into modules that correspond roughly to the emulated computer's subsystems. In our SICvm we have the following modules
A CPU emulator or CPU simulator (the two terms are mostly interchangeable)
A memory subsystem module
The I/O devices emulators
Buses are not emulated in SICvm
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

14. SICvm – A SIC Based Virtual Machine
The CPU Emulator
In any virtual machine this is the most complicated part to emulate
We have implemented an interpreter based CPU Emulation
The logic of the simulated CPU can then more or less be directly translated into software algorithms, creating a software re-implementation that basically mirrors the original hardware implementation.
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

15. The CPU Implementation Details
Each regsiter is emulated using variables
The CPU accomplishes basic fetch using the fetch() function that does 3 types of fetch
Opcode fetch (1 Cycle)
Operand fetch (2 Cycles)
Data fetch (3 Cycles)
Device Code fetch (1 Cycle Byte fetch)
The CPU accomplished storing using a store() fuctions that does the storing of variables from registers to memory locations. This always does a 3 cycle operation
The execute () function executes the instructions
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

16. The CPU Implementation Details (Continued…)
Basic CPU Operation was implemented using the following algorithm
do { check Interrupts; opval = fetch (OPCODE); switch (opval) { case ADD: case MUL: case SUB: case DIV: operand = fetch (opval); execute (ARITHMETIC, opval, operand) break; case (logical instructions): …… case (load-store instructions): …… case (I/O Instructions): …… }
} while (opval != HLT);
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

17. SICvm – A SIC Based Virtual Machine
The Memory Subsystem
It is possible for the memory subsystem emulation to be reduced to simply an array of elements
The SICvm has 1 MB of memory //Memory Declaration unsigned char *mem; //Allocate Memory Dynamically mem = (unsigned char*) malloc (MEMSIZE);
The memory is segmented into System area (from 0x00000 to 0x00FFF) and User area (from 0x01000 to 0xFFFFF)
The memory is accessed by the fetch () and store () fuctions. Direct memory access is also done by STCH and LDCH instructions
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

18. The Memory Subsystem (Continued…)
//Standard fetch function
int fetch (int type) {
int temp;
switch (type) {
//OPCODE fetching Logic
case OPCODE:
get opcode; return opcode;
//OPERAND fetch Logic
case OPERAND:
get operand; return operand;
//DATA fetch Logic
case DATA:
get data; return data;
//Device Code Fetch Logic
case DEVCODE:
get devcode; return devcode; }
return -1;
//Standard store function
void store (int reg) {
int temp;
//Store the 1st 8 bits
temp = reg & 0xFF0000;
temp = temp >> 16;
mem[operand++] = temp;
//Store the 2nd 8 bits
temp = reg & 0xFF00;
temp = temp >> 8;
//Store the 3rd 8 bits
temp = reg & 0xFF;
mem[operand] = temp; }
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

19. The Memory Subsystem (Continued…)
Registers
Memory Address
Contents
1000
454F xxxxxx xxxxxxxx
1010
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx A X L PC SW
OPCODE
OPERAND
DATA
One Cycle Fetch
Two Cycle Fetch
Three Cycle Fetch
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

20. SICvm – A SIC Based Virtual Machine
The I/O Subsystem
The current version of SICvm supports input devices and output devices
The input devices are numbered F1, F2, …
The output devices are numbered 01, 02, ...
Special device 00 is the Boot ROM that contains the bootstrap loader
Special device F0 is standard input (Keyboard)
Special device A0 is standard output (Monitor)
All other devices are represented as files in the running operating system with the prefix “DEV”
Device Access Delays are emulated for actual device behaviour
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

21. The SICvm Architecture
SICvm’s Memory
DEV00
(ROM)
DEVA0
(Monitor)
SICvm
CPU
DEVF0
(Keyboard)
DEV06
DEVF1
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

22. Interrupt Handling In SICvm
All Interrupts in SICvm are generated in the Emulated Hardware Level
Interrupts are generated when errors (like division by zero) are found.
Currently on detecting an interrupt the system goes into a HALT state and prevents further execution
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

23. Interrupts Defined in SICvm
Interrupts in SICvm are defined in “interrupt.h”
INT_OVF //Overflow Interrupt
INT_DIVZERO //Divide by Zero Interrupt
INT_SEGFAULT //Segmentation Fault
INT_INVALID //Invalid Instruction
INT_BADMEM //Insufficient Memory
INT_DEVERR //Bad or Inaccessible Device
INT_BADBOOT //DEV00 Not found
INT_BADLOAD //DEVF1 Not found
INT_OUTOFMEM 8 //Memory Range out of Bounds
INT_NONE //No Interrupts Set
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

24. SICvm – A SIC Based Virtual Machine
The Assembler
The machine code for SIC is generated using a standard 2 Pass assembler as described in Leland L.Beck
In order to support additional features few modifications were made to the assembler
Support for the HLT instruction
Support to generate bootstrap code
Support to generate loader code
Support for Immediate Indexed Addressing
An External OPTAB.TXT for adding extra instructions
The assembler is also accompanied by a macroprocessor if any macro expansion is required
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

25. Essential SIC Softwares
In order for the SIC Virtual Machine to work we needed essential system software
A Bootstrap loader to be placed in DEV00
An Absolute Loader to load normal SIC programs into SICvm’s memory. Placed in DEVF1
A Shell in order to communicate with the SIC hardware in DEVF2
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

26. The Bootstrap Loader Design
The bootstrap loader is based on Beck’s Bootstrap Loader mentioned in System Software
Algorithm: read character while (1) { if (character != EOF) { convert to hex representation move byte to memory location increment location counter } else { goto memory location 0x80 } }
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

27. SICvm – A SIC Based Virtual Machine
The Loader Design
The loader design algorithm is again borrowed from Beck’s System Software
The loader implemented is an absolute loader
Algorithm: read Header record verify Header read First Text record while (record type != ‘E’) { convert character object code to hex form move object code to specified location read next object program record } jump to specified address
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

28. SICvm – A SIC Based Virtual Machine
The Shell Design
In order to make the SIC machine more interactive we designed a minimal shell to interact with the machine
The current shell can interpret 3 commands
Load from a device
Execute the loaded program
Quit the shell
Algorithm: while (1) { get input from keyboard switch (input) { case ‘l’: invoke loader and load from dev break; case ‘e’: jump to currently loaded program address case ‘q’: jump to HLT } }
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

29. The Hierarchical Structure
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

30. SICvm Features Demonstration
Now we will demonstrate various features of SICvm

31. Summary of Various Features
The Machine runs on the standard ASCII set (refer Appendix B of System Software by Leland L.Beck)
The VM currently supports 5 Devices. 1 ROM device (DEV00), 3 Input Devices (DEVF1, DEVF2, DEVF3) and 1 Output Device (DEV06). The machine can work with up to 2 Devices (1 Input and 1 Output) simultaneously.
Dynamic Memory dump of the machine can be shown on the screen.
Supports 1 MB of Memory Space, Out of bounds error detection
Introduced HLT instruction (opcode 0x76) for stopping program after execution.
Introduced Interrupt handling to handle exceptions and error that is detected during the runtime of the program
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

32. Summary of Various Features (Continued…)
Introduction of 2 special devices F0 (standard input) and A0 (standard output) so that SIC machine can use Keyboard and Monitor Devices
Developed a Minimal Shell to communicate with SIC Machine Directly
Introduced Immediate Indexed Mode Addressing
Runs programs in both user mode and supervisor mode
Has the ability to work with 2 Devices (1 Input and 1 Output) Simultaneously
Memory Management, In form of segmentation
Advanced I/O device handling like emulating device delays
Ability to run on both Linux and Windows Platform
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

33. Features Currently Underdevelopment
Introduction of clocking to CPU
Introduction of a display device for SIC
Extension of current machine to support SIC / XE Architecture
A Full fledged working shell / operating environment for the machine
A Dynamically Debuggable interactive interface
A Virtual Machine Monitor for the SICvm
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

34. SICvm – A SIC Based Virtual Machine
Conclusion
Provide developers a quick guide in understanding and developing emulators and virtual machines
Easy Extensibility to support various Exisiting and non existing Machine Architectures
Helping aid for students of computer science and engineering, especially when dealing with Systems programming as they will have an environment for developing and testing programs that are written for SIC
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

35. SICvm – A SIC Based Virtual Machine
References
Leland L.Beck - System Software
Hopcroft and Ullmann - Theory of Computation
Hennesy and Patterson - Computer Organization and Design (Used in Referring General Computer Hardware Organization)
A.K Ray - Advanced Microprocessors (Used In Studying of implementation of Segments in 8086)
Dennis Ritche and Brian W.Kernigham - The C Programming Language (General Programming References)
How to: Writing a computer Emulator (General Emulator Programming References)
Wikipedia – (General Reference)
SICvm – A SIC Based Virtual Machine
sicvm.sourceforge.net

36. Any Questions or Queries
Thank You
Any Questions or Queries