This is a project to build a fictional 16 bit console. It will have a limited amount of memory (lets say 1MB), a simple instruction set to make it easy to program, an easy way to share and load games and a screen keyboard and mouse interface.
The computer will have 1 MB of addressable ram. An interface for a cartridge and non volatile storage and a cpu with a limited speed.
Details still to be determined but I am thinking of having a very simple instruction set.
A full instruction consists of (except for jmp) two bytes. The first specifies the instruction used and the second byte are the arguments of the instruction
There are 4 general purpose registers ra rb rc and rd. These registers can be used for anything. There are two special purpose registers, sp the stack pointer and of, the offset register.
| Register | Opcode |
|---|---|
| ra (general purpose) | 000 (0x0) |
| rb (general purpose) | 001 (0x1) |
| rc (general purpose | 010 (0x2) |
| rd (general purpose) | 011 (0x3) |
| sp (stack pointer) | 100 (0x4) |
| of (offset register) | 101 (0x5) |
| ip (instruction pointer) | 110 (0x6) |
| unused | 111 (0x7) |
The width of the opcode is four bits but the opcode shown only takes up three. The first bit specifies whether or not the data at the memory address pointed by the register is used or if the value of the address itself will be used. for example
0000 0011 1000 0001 00000 0000 0000 0000 or in assembly move [ra] rb moves the data pointed by ra to rb.
| Instruction | Opcode | Description | Arguments |
|---|---|---|---|
| term | 0000 0000 (0x00) | Halts the machine | None |
| push | 0000 0001 (0x01) | pushs address onto stack | 1 byte last three bytes contain register |
| pop | 0000 0010 (0x02) | pops address of of stack | 1 byte last three bytes contain register |
| move | 0000 0011 (0x03) | moves data | 1 byte first 4 bits dest last 4 source |
| movc | 0000 0100 (0x04) | moves constant into register | 1st byte register 2 bytes constant |
| jump | 0000 0101 (0x05) | jumps to memory address | 2 bytes address to jump too |
| call | 0000 0110 (0x06) | calls address | 2 butes address to jump too |
| ret | 0000 0111 (0x07) | pops address of of stack into ip | None |
| addu | 0000 1000 (0x08) | adds two registers together | 1 byte first 4 bits dest last 4 source |
| adds | 0000 1001 (0x09) | adds signed | 1 byte first 4 bits dest last 4 source |
| sub | 0000 1010 (0x0A) | subtracts to registers | 1 byte first 4 bits dest last 4 source |
| je | 0000 1011 (0x0B) | jump if equal | 1 byte source and dest registers |
| jg | 0000 1100 (0x0C) | jump if source is greater | 1 byte source and dest registers |
| jl | 0000 1101 (0x0D) | jump if source is less | 1 byte source and dest registers |
| int | 0000 1110 (0x0E) | Hardware interrupt | 2 bytes interrupt number |
| intr | 0000 1111 (0x0F) | Hardware interrupt register | 1 byte register containing interrupt |
Inorder to access the full range of memory sp is used as the first 4 bytes of the regiser. a typical access operation looks like:
sp | register used
0000 0000 00000 MEMORY | 0110 1101 1110 1111
This just terminates the machine.
Opcode:
0000 0000 0000 0000 0000 0000 0000 0000
Pushes reg onto stack. sp is incremented by two bytes (by two)
opcode:
0000 0001 0000 [reg] 0000 0000 0000 0000
Pops data from stack into register. sp is decremented by two
opcode:
0000 0010 0000 [reg] 0000 0000 0000 0000
Moves data from one register into another register
opcode:
0000 0011 [dest reg][src reg] 0000 0000 0000 0000
Moves constant value into a register
opcode:
0000 0100 0000 [dest reg] [constant value 2 bytes]
jumps to specified address. It sets the ip to the address specified.
opcode:
0000 0101 [2 bytes dest address] 0000 0000
The address used by jump is the index of the first byte of the isntruction jumped to divided by 4. 4 is the width of all opcodes
jumps to a specified address and then pushes next instruction on stack. It pushes the current value of ip+=1
opcode:
0000 0110 [2 bytes dest address] 0000 0000
jumps to instruction pushed off of stack. It pushes a value off of the stack into ip and goes to it.
opcode:
0000 0111 0000 0000 0000 0000 0000 0000
adds two unsigned registers together. result is stored in dest
opcode:
0000 1000 [dest] [src] 0000 0000 0000 0000
adds two signed registers together. result is stored in dest
opcode:
0000 1001 [dest] [src] 0000 0000 0000 0000
Jumps if two registers are equal
opcode:
0000 1011 [reg1] [reg2] [address to jump to (2 bytes)]
Jumps if reg1 is greater than reg2
opcode:
0000 1100 [reg1] [reg2] [address to jump to (2 bytes)]
Jumps if reg1 is less than reg2
opcode:
0000 1101 [reg1] [reg2] [address to jump to (2 bytes)]
Performs a hardware interrupt. See docs/interrupt.md for more details about hardware interrupts.
Opcode
0000 1110 [2 bytes interrupt code] 0000 0000
Performs a hardware interrupt based on value in register.
Opcode
0000 1111 0000 [register] 0000 0000