R0 is a hard-wired 8-bit CPU, built on Von Neumann Architecture.
It was built as an application on the theory of Computer Organization.
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Half of the instructions in the instruction set fit into one byte.
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These instructions are identified by a 0 in the most-significant bit in the instruction, i.e. op1 = 0X.
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The 4 bits of opcode are split into op1 and op2.
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Rd is the destination register, and Rs is the source register.
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The other half of the instruction set are two-byte instructions. The first byte has the same format as above, and it is followed by an 8-bit constant or immediate value
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These two-byte instructions are identified by a 1 in the most-significant bit in the instruction, i.e. op1 = 1X.
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With 4 operation bits, there are 16 instructions.
| op1 | op2 | Mnemonic | Purpose |
|---|---|---|---|
| 00 | 00 | AND | Rd, Rs Rd = Rd AND Rs |
| 00 | 01 | OR | Rd, Rs Rd = Rd OR Rs |
| 00 | 10 | ADD | Rd, Rs Rd = Rd + Rs |
| 00 | 11 | SUB | Rd, Rs Rd = Rd - Rs |
| 01 | 00 | LW | Rd, (Rs) Rd = Mem[Rs] |
| 01 | 01 | SW | Rd, (Rs) Mem[Rs] = Rd |
| 01 | 10 | MOV | Rd, Rs Rd = Rs |
| 01 | 11 | NOP | Do nothing |
| 10 | 00 | JEQ | Rd, immed PC = immed if Rd == 0 |
| 10 | 01 | JNE | Rd, immed PC = immed if Rd != 0 |
| 10 | 10 | JGT | Rd, immed PC = immed if Rd > 0 |
| 10 | 11 | JLT | Rd, immed PC = immed if Rd < 0 |
| 11 | 00 | LW | Rd, immed Rd = Mem[immed] |
| 11 | 01 | SW | Rd, immed Mem[immed] = Rd |
| 11 | 10 | LI | Rd, immed Rd = immed |
| 11 | 11 | JMP | immed PC = immed |
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The CPU has an 8-bit data bus and an 8-bit address bus, so it can only support 256 bytes of memory to hold both instructions and data.
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Internally, there are four 8-bit registers, R0 to R3, plus an Instruction Register, the Program Counter, and an 8-bit register which holds immediate values.
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The ALU is the same one that we designed last week. It performs the four operations AND, OR, ADD and SUB on two 8-bit values, and supports signed ADDs and SUBs.
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The CPU is a load/store architecture: data has to be brought into registers for manipulation, as the ALU only reads from and writes back to the registers.
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The ALU operations have two operands: one register is a source register, and the second register is both source and destination register, i.e. destination register = destination register OP source register.
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All the jump operations perform absolute jumps; there are no PC-relative branches. There are conditional jumps based on the zeroness or negativity of the destination register, as well as a "jump always" instruction.
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The dbus and sbus labels indicate the lines coming out from the register file which hold the value of the destination and source registers.
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Note the data loop involving the registers and the ALU, whose output can only go back into a register.
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The dataout bus is only connected to the dbus line, so the only value which can be written to memory is the destination register.
This Project is signed under MIT License.