XenoM-Processor
A digital design of a five stage pipelined RISC processor.
Processor Default Specifications
- The data cache address space is 1 MB of 16-bit width and is word addressable. (Note: word = 2 bytes).
- There are eight 2-byte general purpose registers; R0, till R7.
- There are three special purpose registers
- Program counter (PC).
- Exception program counter (EPC).
- Stack pointer (SP).
- The initial value of SP is (2^20-1).
- This project handles two types of exceptions
- Empty Stack Exception which occurs when a pop is called while the stack is empty.
- Invalid memory address which occurs when the address exceeds the range of 0xFF00.
- The exception handler address for empty stack exception is stored in the instruction cache in locations M[2] and M[3].
- The exception handler address for invalid mem address exception is stored in the instruction cache in locations M[4] and M[5].
- When an exception occurs, the address of the instruction causing the exception is saved in the exception program counter (EPC).
- There are 3 flags; implemented in a condition code register named flags<3:0>
- Zero flag, flags<0>, which changes after arithmetic, logical, or shift operations.
- Negative flag, flags<1>, which changes after arithmetic, logical, or shift operations
- Carry flag, flags<2>, which changes after arithmetic or shift operations.
- There is one input port named that takes a value from the port and write it in a register.
- There is one output port that takes a value from a register and out it to the port.
- There is a Reset signal named reset in the integration.vhd
Instruction Set Architecture
| Mnemonic | Function |
|---|---|
| NOP | PC ← PC + 1 |
| HLT | Freezes PC until a reset |
| SETC | C ← 1 |
| NOT Rdst | NOT value stored in register Rdst R[Rdst] ← 1’s Complement(R[Rdst]); If (1’s Complement(R[Rdst]) = 0): Z ← 1; else: Z ← 0; If (1’s Complement(R[Rdst]) < 0): N ← 1; else: N ← 0 |
| INC Rdst | Increment value stored in Rdst R[Rdst] ← R[Rdst] + 1; If ((R[Rdst] + 1) = 0): Z ← 1; else: Z ← 0; If ((R[Rdst] + 1) < 0): N ← 1; else: N ←0 Updates carry |
| OUT Rdst | OUT.PORT ← R[Rdst] |
| IN Rdst | R[Rdst] ← IN.PORT |
| MOV Rsrc, Rdst | Move value from register Rsrc to register Rdst |
| ADD Rdst, Rsrc1, Rsrc2 |
Add the values stored in registers Rsrc1, Rsrc2 and store the result in Rdstand updates carry If the result = 0 then Z ← 1; else: Z ← 0; If the result < 0 then N ← 1; else: N ← 0 |
| SUB Rdst, Rsrc1, Rsrc2 |
Subtract the values stored in registers Rsrc1, Rsrc2 and store the result in Rdst and updates carry If the result = 0 then Z ← 1; else: Z ← 0; If the result < 0 then N ← 1; else: N ← 0 |
| AND Rdst, Rsrc1, Rsrc2 |
AND the values stored in registers Rsrc1, Rsrc2 and store the result in Rdst If the result = 0 then Z ← 1; else: Z ← 0; If the result < 0 then N ← 1; else: N ← 0 |
| IADD Rdst, Rsrc ,Imm |
Add the values stored in registers Rsrc to Immediate Value and store the result in Rdst and updates carry If the result = 0 then Z ← 1; else: Z ← 0; If the result < 0 then N ← 1; else: N ← 0 |
| PUSH Rdst | X[SP] ← R[Rdst]; SP-=1 |
| POP Rdst | SP+=1; R[Rdst] ← X[SP]; |
| LDM Rdst, Imm | Load immediate value (16 bit) to register Rdst R[Rdst] ← Imm<15:0> |
| LDD Rdst, offset(Rsrc) |
Load value from memory address Rsrc + offset to register Rdst R[Rdst] ← M[R[Rsrc] + offset]; |
| STD Rsrc1, offset(Rsrc2) |
Store value that is in register Rsrc1 to memory location Rsrc2 + offset M[R[Rsrc2] + offset] ←R[Rsrc1]; |
| JZ Rdst | Jump if zero If (Z=1): PC ← R[Rdst]; (Z=0) |
| JN Rdst | Jump if negative If (N=1): PC ← R[Rdst]; (N=0) |
| JC Rdst | Jump if carry If (C=1): PC ← R[Rdst]; (C=0) |
| JMP Rdst | Jump PC ← R[Rdst] |
| CALL Rdst | X[SP] ← PC + 1; sp-=2; PC ← R[Rdst] |
| RET | sp+=2, PC ← X[SP] |
| INT index | X[SP] ← PC + 1; sp-=2;Flags reserved; PC ← M[index + 6] & M[index + 7] index is either 0 or 2. |
| RTI | sp+=2; PC ← X[SP]; Flags restored |