4 ‐ Instructions and Codes

For this section, the mnemonics and abstractions used will be those implemented in Fox Assembly. However, a different implementation of an assembly language for this architecture could handle things differently. Such alternative implementations, again, would be welcomed. Additionally, all opcodes or operands being 64 bits in-size leaves room for extensions and modifications to the architecture, and thus the number of instructions will not change in the future, to encourage such endeavours.

As to how opcodes, operands, and data are stored in the binary format used by Fox, everything is, again, of a size matching the 64 bit word-size of the architecture overall. Each opcode, operand, or other datum are stored in a space of 64 bits each. Operands simply follow the opcode in the next arbitrary number of 64 bit slots, for the amount required for that opcode. For example, the instruction a+, which requires no operands, would only take up one 64 bit slot. However, the instruction mv mdr acc would use three 64 bit slots, one for mv, the next for mdr, and the next for acc.

Instructions

Opcode	Mnemonic	Operands	Description
0	pl	value register	Place a raw value into register
1	mv	register1 register2	Move the contents of register1 into register2
2	st		Store MDR at MAR in the selected MCH
3	ld		Load into MDR the data at MAR in the selected MCH
4	jm	address	Set CEA to address
5	js	address	Set CEA to address if Accumulator is set (non-zero)
6	jc	address	Set CEA to address if Accumulator is clear (zero)
7	a+		Add the contents of Data to Accumulator and store the result in Accumulator
8	a-		Subtract the contents of Data from Accumulator and store the result in Accumulator
9	a!		Invert the bits of the Accumulator
10	ai		Increment the Accumulator by 1
11	ad		Decrement the Accumulator by 1
12	a*		Multiply the contents of Accumulator by Data and store the result in Accumulator
13	a/		Divide the Accumulator by Data and store the result in Accumulator
14	a&		Logically AND the bits of Accumulator with the bits of Data and store the result in Accumulator
15	a|		Logically OR the bits of Accumulator with the bits of Data and store the result in Accumulator
16	a^		Logically XOR the bits of Accumulator with the bits of Data and store the result in Accumulator
17	al		Left shift bits of Accumulator by 1, Data amount of times
18	ar		Right shift bits of Accumulator by 1, Data amount of times
19	gt		Set the Accumulator to non-zero if it is greater than Data, otherwise set it to zero
20	lt		Set the Accumulator to non-zero if it is less than Data, otherwise set it to zero
21	ge		Set the Accumulator to non-zero if it is greater than or equal to Data, otherwise set it to zero
22	le		Set the Accumulator to non-zero if it is less than or equal to Data, otherwise set it to zero
23	eq		Set the Accumulator to non-zero if it is equal to Data, otherwise set it to zero
24	ne		Set the Accumulator to non-zero if it is not equal to Data, otherwise set it to zero
25	cl	address	Jump to address after pushing CEA onto the Callstack
26	rt		Jump to the address on the Callstack at CSP, then pop this address off of the Callstack
27	fi		Terminate program execution and initiate the shutdown process (should be included at the end of all programs that are expected to reach the end of their execution)

Codes

Memory Channel	Mnemonic	Description
0	mem	Main Memory
1	inp	Input
2	out	Output
3	cst	Callstack

Registers gp0-gp7 are implemented as of v0.8-beta:

Register	Mnemonic	Description
0	mch	Memory Channel
1	mar	Memory Address Register
2	mdr	Memory Data Register
3	acc	Accumulator
4	dat	Data
5	cea	Current Execution Address
6	csp	Callstack Pointer
7	gp0	General Purpose Register 0
8	gp1	General Purpose Register 1
9	gp2	General Purpose Register 2
10	gp3	General Purpose Register 3
11	gp4	General Purpose Register 4
12	gp5	General Purpose Register 5
13	gp6	General Purpose Register 6
14	gp7	General Purpose Register 7

Memory-Channel-Specific Addresses

If Input or Output is the selected Memory Channel, the possible values for MAR mean different things.

Value of MAR	Area Written-to or Read-from when Input or Output is the current Memory Channel
0	Standard I/O
1	Secondary Storage
2	Screen Buffer (implemented as of v0.4-alpha)
3	Keyboard (implemented as of v0.8-beta)
4+	Currently unimplemented

Fox Assembly-Specific Syntaxes

Syntax	Meaning
label= ...	Replace all instances of label with ... for the entirety of the source document
label: label: ...	Replace all instances of label with the address of the data that it is placed before for the entirety of the source document
[text]s	Store the bytes of text from this point downwards, each byte at the lowest byte of their own 64 bit cell
[number]d	Store the 64-bit base-10 (decimal) number at this point
[number]x	Store the 64-bit base-16 (hexadecimal) number at this point
[number]o	Store the 64-bit base-8 (octal) number at this point
[number]b	Store the 64-bit base-2 (binary) number at this point
; ...	Do not process anything between the ; and a newline or end-of-file

Things to watch out for

Fox Assembly uses whitespace to separate tokens. Any amount of whitespace can be used, but it must be included. For example, zero= [0]b is valid syntax, but zero=[0]b is not, and nor is zero =[0]b, nor zero = [0]b.

At this point in time, hexadecimal numbers must use uppercase letters and not lowercase letters. This will be changed in-future, but for now something like [5D]x is valid whilst [5d]x is not. This is amended as of v0.3-alpha!