( This project can be implemented in groups of at most two students.)
This project is to be done with Python. In this project, you will implement (i) an assembler and (ii) an execution simulator for a hypothetical CPU called CPU230. CPU230 is illustrated in the following figure:
Each instruction has fixed length of 3 bytes with the following format:
| Opcode | Adressing mode | Operand |
| 6 bits | 2 bits | 16 bits |
Addressing mode bits are as follows:
| Bits(binary) | Addressing mode |
| 00 | operand is immediate data |
| 01 | operand is in given in the register |
| 10 | operands memory address is given in the register |
| 11 | operand is a memory address |
| Note that registers are represented as bit patterns (here given in hex): PC=0000, A=0001, B=0002, C=0003, D=0004, E=0005, S=0006. | |
Instructions are as follows:
| Instruction | Instruction code (hex) | Operand | Meaning | Flags set |
| HALT | 1 | Halts the CPU. | ||
| LOAD | 2 | immediate memoryregister | Loads operand onto A . | |
| STORE | 3 | memoryregister | Stores value in A to the operand. | |
| ADD | 4 | immediate memoryregister | adds operand to A. | CF,SF, ZF |
| SUB | 5 | immediate memoryregister | subtracts operand from A. | CF,SF, ZF |
| INC | 6 | immediate | increments operand (equivalent to add 1) | SF, ZF, CF |
| memoryregister | ||||
| DEC | 7 | immediate memoryregister | decrements operand (equivalent to sub 1) | SF, ZF, CF |
| XOR | 8 | immediate memoryregister | Bitwise XOR operand with A and store result in A. | SF, ZF |
| AND | 9 | immediate memoryregister | Bitwise AND operand with A and store result in A. | SF, ZF |
| OR | A | immediate memoryregister | Bitwise OR operand with A and store result in A. | SF, ZF |
| NOT | B | immediate memoryregister | Take complement of the bits of the operand. | SF, ZF |
| SHL | C | register | Shift the bits of register one position to the left. | SF, ZF, CF |
| SHR | D | register | Shift the bits of register one position to the right. | SF, ZF |
| NOP | E | No operation. | ||
| PUSH | F | register | Push a word sized operand (two bytes) and update S by subtracting 2. | |
| POP | 10 | register | Pop a word sized data (two bytes) into the operand and update S by adding 2. | |
| CMP | 11 | immediate memoryregister | Perform comparison (AC-operand) and set flag accordingly. | SF, ZF, CF |
| JMP | 12 | immediate | Unconditional jump. Set PC to address. | |
| JZ JE | 13 | immediate | Conditional jump. Jump to address (given as immediate operand) if zero flag is true. | |
| JNZ JNE | 14 | immediate | Conditional jump. Jump to address (given as immediate operand) if zero flag is false. | |
| JC | 15 | immediate | Conditional jump. Jump if carry flag is true. | |
| JNC | 16 | immediate | Conditional jump. Jump if carry flag is false. | |
| JA | 17 | immediate | Conditional jump. Jump if carry flag is false. | |
| JAE | 18 | immediate | Conditional jump. Jump if above or equal. | |
| JB | 19 | immediate | Conditional jump. Jump if below. | |
| JBE | 1A | immediate | Conditional jump. Jump if below or equal. | |
| READ | 1B | memoryregister | Reads a character into the operand. | |
| 1C | immediate memoryregister | Prints the operand as a character. |
Note that memory address can be given as [xxxx] or [r] where xxxx is a hexadecimal number or r where r is a register name.
Labels can also be used. A label: marks the address, xxxx, at the point it is defined. Wherever you use a label, you should substitute the marked address xxxx for the label.
The assembler you build will be called cpu230assemble and the execution simulator will be called cpu230exec. They will be used as follows. Suppose you are given a assembly program given in file prog.asm. The following command will assemble the program and produce the binary output prog.bin.
> cpu230assemble prog.asm
The following program will execute the binary
> cpu230exec prog.bin
The above process is illustrated in the example below:
| Assembly source code: prog.asm | Assemble | Assembled program : prog.bin | Execute | Output |
| LOAD A STORE C LOAD MYDATA STORE B LOAD 0004 STORE D LOOP1: PRINT C LOAD C STORE [B] INC C INC B INC B DEC D JNZ LOOP1 HALT MYDATA: | cpu230assemble prog.asm | 080041 0D0003 08002D 0D0002 080004 0D0004 710003 090003 0E0002 190003 190002 190002 1D0004 530012 040000 | cpu230exec prog.bin | A B C D |
Note also that in the above example, ascii codes of A, B, C, D and E are stored at the memory addresses 002D, 002F, 0031, 0033, 0035.
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