SpaceCat Assembly Simulator

SpaceCat Assembly Simulator is an educational tool that simulates an 8-bit processor as defined in Computer Science: An Overview by Brookshear & Brylow. It is compatible with a previous implementation of the same machine called SimpSim (or BasSim, depending on the locale.)

The Machine

SVM is a simple machine, it contains 256 memory addresses each holding up to 32 bits of data, as well as 16 registers.

Special Registers

Name	Representation	Usage
Program Counter	PC	Program counter holds the address of the next memory cell to execute
Instruction Register	IR	Instruction register holds the last instruction that was executed.
STDOUT	RF	RF register is mapped to the STDOUT, when RF is modified in some way, character corresponding to the value inside RF according to RF is outputted to STDOUT, which is almost always the screen.

Instructions and Assembly Language

SVM works on a simplified assembly language.

Data

In SVM, as in the previous implementation, there are integers, and only* integers.

the Integer

Name	Representation	Example
Decimal	-D, Dd, D	-10, 10d, -10d, 10
Binary	Db	10101b
Hexadecimal	0xD, Dh, $D	0xAA, AAh, $AA

The Float

The floating point numerals are represented as proposed in the book Computer Science: An Overview, this representation is... close to IEEE 754, however, there are differences. To start our journey, it is important to understand that

• Assembly Language as used in SVM does not formally define floating point numbers, when two values are combined using ADDF instruction, they are interpreted as Floating point numerals.

• SVM's floating point numerals are 8-bit floating point numerals sometimes lovingly called minifloats

• They are structured as follows

Sign Bit	Exponent	Mantissa
1 bit	4 bits	3 bits
one for negative, zero for positive	balanced for 1000 = 0	The mantissa.

And then calculated as for exponent = e, mantissa = m 2^em.

Base Instructions

Instruction	Operands	Explanation	OP-Code Representation
LOAD	Register, [Memory Address]	Loads the value in the memory address to register	1RMe
LOAD	Register, Value	Loads the value to the register	2RVa
STORE	Register, [Memory Address]	Store the value in register in the memory address	3RMe
MOVE	Register, Register	Move the value in register 1 to register 2	40RR
ROR	Register, Value	Rotate the Register right Value times	AR0V
JMP	Value	Jump to the memory cell indexed value	B0Va
JMPEq	Register =R0, Value	Jump to Value index if Register equals R0	BRVa
HALT	No operands.	Halt the program execution, for SVM this mostly means stopping the program	C000
LOAD	Register, Register	Load the value in the memory index hold by the second register to the first register	D0RR
STORE	Register, Register	Store the value in register one to the memory index hold in second register.	E0RR
JMPLe	Register <=R0, Value	Jump to the value index if the value in register is less then the one in R0.	FRVa

Arithmatic Instructions

Instructions which always take three registers as operands, applies an operation to values in second and third registers and stores the result in the first register.

Mnemonic	Operation	OP-Code Representation
ADDI	Integer addition	5RRR
ADDF	Floating point addition	6RRR
OR	Bitwise or	7RRR
AND	Bitwise and	8RRR
XOR	Bitwise exclusive or	9RRR

Directives

Assembly language used by SVM also has a directives. Directives may change the assembled memory, but they do not get converted into an OP-Code.

Directive	Usage	Effect
ORG	Org Address	Tells the assembler that the succeeding instruction will be put into the memory adress.
DB	DB String	Tells the assembler to put the ASCII values of the characters in string to memory cells, one character at a time.

Labels

Labels are pretty much pointers, they denote a place in the memory, they can be assigned to a DB directive or to the start of a line, afterwards, they reefer to the address of the following line.

Comments

Start of a comment statement in SVM Assembly is denoted by a ; and its end is denoted by the end-of-line character \n. Comments are ignored by the assembler.

On SpaceCat

Frequently Asked Questions

What are the requirements for SpaceCat?

SpaceCat requires at least Python 3.8 due to its use of typing hints (3.5+) and the walrus operator (3.8+) in some places, it also requires tkinter package for its GUI.

How compatible is it with SimpSim/BasSim?

Very compatible, it can do most of the stuff SimpSim does and aims for total coverage very soon, moreover, it supports the *.prg file format, which is what SimpSim exports its memory.

What is a machine state?

It is a new addition, rather than only dumping the memory, it saves the registers as well, so it can freeze the machine state and save it.

Why Python?

SVM is implemented in Python 3.8, it can therefore run in any platform supporting 3.8, but it also offers binaries for Windows. It probably can run on PyPy as well.

The missing stuff

• Turkish translation.
• Editable cells.
• OctalFloat might not be working as intended, I can't wrap my head around additions and truncations.