CPU Architecture ​

Features ​

• 16bit data & address width
• Software stack
• Von Neumann architecture for loading instructions

Inspirations ​

• x86_64 (registers)
• jdah/jdh-8 (spec, some instructions)

Registers ​

The cpu will have 16 user registers, r0-9, ra-rd and stack registers. (rsp and rbp - stack and base pointer) In the future I might consider adding rsa and rdi if I add interrupts.

In addition to user accessible registers, there will be a flags register and a status register, which are not directly accessible.

Flags register ​

Status register ​

| Value | Status | | 0x0 | Off | | 0x1 | Power | | 0x2 | Halt |

Instructions ​

• halt -> status = Halt(0x2)
• noop -> skip cycle
• mov reg, reg/imm16 -> reg = reg/imm16 (move value into register)
• ldm reg, [reg/imm16] -> reg = mem[reg/imm16] (load from memory)
• sim [reg/imm16], reg/imm16 -> mem[reg/imm16] = reg/imm16 (store into memory)
• push reg/imm16 -> rsp++; mem[rsp] = reg/imm16 (push value into stack)
• pop reg -> reg = mem[rsp]; rsp-- (pop value off stack)
• jmp reg/imm16 -> pc = reg/imm16 (Jump to address, jc, jnc, jz, jnz, js, jns, jo and jno)
• set(f) -> flags = Mask[f] | 1 << Mask[f] (set flag, f = flag)
• clr(f) -> flags = Mask[f] & ~Mask[f] (clear flag, f = flag)

ALU Instructions ​

• not reg -> reg = ~reg (bitwise not)
• or reg, reg/imm16 -> reg = reg | reg/imm16 (bitwise or)
• xor reg, reg/imm16 -> reg = reg ^ reg/imm16 (bitwise xor)
• and reg, reg/imm16 -> reg = reg & reg/imm16 (bitwise and)
• add reg, reg/imm16 -> reg = reg + reg/imm16 (add)
• addc reg, reg/imm16 -> reg = reg + reg/imm16 + c (add with carry)
• sub reg, reg/imm16 -> reg = reg - reg/imm16 (subtract operation)
• subb reg, reg/imm16 -> reg = reg - reg/imm16 - c (subtract with carry)
• inc reg -> reg = reg + 1 (increment)
• dec reg -> reg = reg - 1 (decrement)
• shl reg, reg4/imm4 -> reg = reg << reg4/imm4 (shift left)
• shr reg, reg4/imm4 -> reg = reg >> reg4/imm4 (shift right)
• rotl reg, reg4/imm4 -> reg = reg rotate left by reg4/imm4 (rotate left)
• rotr reg, reg4/imm4 -> reg = reg rotate left by reg4/imm4 (rotate right)
• mul reg8, reg8/imm8 -> reg = reg8 * reg8/imm8 (multiply)

imm4/8/16 are either values immediately after the instruction, or in case of imm4 can be inside the instruction.