Instruction Set

The ByteFrost computer uses a RISC-inspired instruction set. Currently, it is designed to have a maximum of 32 different opcodes (hence opcodes take 5 bits in an instruction). Each instruction is composed of 2 bytes.

Each instruction must specify the opcode to be used (the opcode goes in the 5 least-significant bits of the first byte of an instruction), and can specify a destination register, a maximum of two source registers, and / or an immediate. There are other variations specific to particular instructions, but these are the general constraints. To simplify hardware implementation, the instructions are structured as similar to each other as possible - for instance, the location of the destination register (Rd) will always be in bits 6 and 7 of the overall instruction.

Here are two general instruction skeletons that illustrates this structure:

15 14	13 12	11 10 9 8	7 6	5	4 3 2 1 0
Rs2	Rs1	X X X X	Rd	X	Opcode

15 14 13 12 11 10 9 8	7 6	5	4 3 2 1 0
8-bit immediate	Rd	X	Opcode

A list of currently implemented instructions follows:

Opcode	Instruction	Use	Assembly Syntax	Sets Flags	Structure
0	NOP	No operation	NOP	No	0x00 0x00
1	BRK	Halts operation	BRK	No	0x01 0x00
2	ALU	Use ALU to modify values in two input registers and store output in a register	(ALU function) Rd, Rs1, Rs2	Yes	[Rd X 00010] [Rs2 Rs1 Function select]
3	LDR	Load immediate value to a register	LDR Rd, #Imm	No	[Rd X 00011] [ 8-bit immediate]
4	MOV	Copy value in one register to another	MOV Rd, Rs1	No	[Rd X 00100] [XX Rs1 XXXX]
5	Branch immediate	Conditionally sets program counter to an immediate value	(Branch condition) #Imm	No	[Condition 00101] [8-bit immediate]
6	ALU immediate	Use ALU taking the value of a source register and an immediate, then setting the source register to the new value	(ALU function) Rd, #Imm	Yes	[Rd X 00110] [4-bit immediate Function select]
7	Branch relative	Conditionally adds an immediate value to the program counter and sets the program counter to the new value	(Branch condition) +/-Imm	No	[Condition 00111] [8-bit immediate]
8	OUT	Prints value of a register to the display (can print the int value as is or the ASCII character)	OUT Rs1, A/I	No	[XX A/I 01000] [XX Rs1 XXXX]
9	LMA	Load a value from memory at a given address to a register	LMA Rd, #Imm	No	[Rd X 01001] [8-bit immediate]
A	SMA	Store a value in a register to memory at a given address	SMA Rd, #Imm	No	[Rd X 01010] [8-bit immediate]
B	LMR	Load a value from memory at an address stored in a register to another register	LMR Rd, Rs1	No	[Rd X 01011] [XX Rs1 XXXX]
C	SMR	Store a value in a register to memory at an address stored in another register	SMR Rd, Rs1	No	[Rd X 01100] [XX Rs1 XXXX]
D	OUT immediate	Prints an immediate value to the display (can print the int value as is or the ASCII character)	OUT #Imm, A/I	No	[XX A/I 01101] [8-bit immediate]
E	PUSH	Store a value in a register to the stack and increment stack pointer	PUSH Rs1	No	[XXX 01110] [XX Rs1 XXXX]
F	POP	Decrement stack pointer and retrieve value from the stack to a register	POP Rd	No	[Rd X 01111] [XXXX XXXX]
10	JSR	Jump to subroutine (store next program address on the stack)	JSR #Imm	No	[XXX 10000] [8-bit immediate]
11	RTS	Return from subroutine (load PC with return address on the stack)	RTS	No	[XXX 10001] [XXXX XXXX]
12	TEST	Use ALU to modify values in two input registers but doesn't save output (used to update flags)	COMP Rs1, Rs2	Yes	[XXX 10010] [Rs2 Rs1 Function select]
13	TEST immediate	Use ALU to modify values in an input register and immediate but doesn't save output (used to updated flags)	COMP Rs1, #Imm or TST Rs1	Yes	[XXX 10011] [4-bit immediate Function select]