时间：2023-06-30 20:36:01 | 来源：网站运营
时间：2023-06-30 20:36:01 来源：网站运营
LC-3虚拟机: C语言实现：
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a+b 这件事， 并且除了 PC、 SP 外没有涉及其他的寄存器。int main(){ content = read_binary_file("2048.obj"); reg[R_PC] = 0x3000; signal(SIGINT, handle_interrupt); disable_input_buffering(); while (1) { instruction = mem_read(reg[R_PC]); reg[R_PC]++; ret = execute(instruction); if (ret != 0) break; } restore_input_buffering(); return 0;}sign_extend() 函数的实现以及多处调用， 看起来挺美好， 但如果稍微不听话不完全按 tutorial 来， 容易踩坑：sign_extend() 可能写错参数sign_extend() 函数的地方有多处， 而每次调用都需要传入两个参数： 一个是 mask， 另一个是位数， 这两个参数其实只用第二个就可以表达第一个， 因此有点冗余； 而多次调用这样的冗余表达的代码， 一不小心少写一个 F 就导致在 debug 上浪费几十分钟（想想看， 2048的界面迟迟跑不出来）。sign_extend() 更大的坑在于， 如果不打算用原文 tutorial 中对于内存读写封装的 mem_write() 函数而是直接写入表示内存的数组， 会发生内存越界。sign_extend()返回值是 uint16_t 类型， 通常作为 offset 存在， 和 pc 相加后作为一个内存地址， 然后从 memory 数组中获取元素， 即:uint16_t dr = (instr >> 9) & 0x7; uint16_t pc_offset = sign_extend(instr & 0x1ff, 9); mem_write(reg[R_PC] + pc_offset, reg[dr]);这里的问题在于： 如果 instr 最后9位全是1， 那么 sign_extend 后得到的是 65535， 而不是 -1； 而 65535 和 reg[R_PC] 相加， 由于这两个都是 uint16_t 类型的， 那么肯定是会大于 65535。mem_write() 函数， 而是直接用 memory[reg[R_PC] + pc_offset] = reg[dr]， 就会导致越界。uint16_t sign_extend(uint16_t x, int bit_count) { if ((x >> (bit_count - 1)) & 1) { x |= (0xFFFF << bit_count); } return x;}实际上这里无论是输入参数还是返回值， 用 int16_t 类型都是更加妥当的, 避免了传参和接收结果时的数据类型隐式转换， 降低了心智负担， 消除了不必要的内存越界问题：int16_t sign_extend(int16_t x, int bit_count){ if ((x >> (bit_count - 1)) & 1) { x |= (0xFFFF << bit_count); } return x;}all: clang main.c #-fno-omit-frame-pointer -fsanitize=address.PHONY: runrun: ./a.out ./2048.obj遇到了几个问题， 稍作记录：make: *** [makefile:7：run] 错误 254(base) zz@localhost% ./a.out ./2048.obj Control the game using WASD keys.zsh: segmentation fault (core dumped) ./a.out ./2048.obj而通过 make run 执行得到的报错则是:(base) zz@localhost% make run./a.out ./2048.objControl the game using WASD keys.make: *** [makefile:7: run] 段错误 (核心已转储)sign_extend() 函数中所提到的， 最开始用 uint16_t 作为参数和返回值类型， 并且直接写入 memory 数组时由于索引超过 65535 （16bit最大数字）导致越界， 但开启 ASAN 时并没有报告出来。 原因暂时不明。// The LC-3 VM implementation// based on https://www.jmeiners.com/lc3-vm/#include <stdint.h>#include <stdlib.h>#include <stdio.h>#include <stdbool.h>#include <signal.h>#if __linux__ || __APPLE__#include <unistd.h>#include <termios.h>#elif _MSC_VER#include <windows.h>#include <conio.h>#endif//#include "px_log.h"#if __ANDROID_API__ >= 8#include <android/log.h>#define PX_LOGE(...) do { / fprintf(stderr, ##__VA_ARGS__); fprintf(stderr, "/n"); / __android_log_print(ANDROID_LOG_WARN, "pixel", ##__VA_ARGS__); } while(0)#else // __ANDROID_API__ >= 8#include <stdio.h>#define PX_LOGE(...) do { / fprintf(stderr, ##__VA_ARGS__); fprintf(stderr, "/n"); } while(0)#endif // __ANDROID_API__ >= 8uint16_t memory[UINT16_MAX];int running = 1;enum{ R_R0 = 0, R_R1, R_R2, R_R3, R_R4, R_R5, R_R6, R_R7, R_PC, R_COND, // 是 PSR(Processor Status Register)寄存器的低3位, 即 PSR[2:0] R_COUNT};// registersuint16_t reg[R_COUNT];// opcode definitionenum{ OP_BR = 0, // branch OP_ADD, // add OP_LD, // load OP_ST, // store OP_JSR, // jump register OP_AND, // bitwise and OP_LDR, // load register OP_STR, // store register OP_RTI, // unused OP_NOT, // bitwise not OP_LDI, // load indirect OP_STI, // store indirect OP_JMP, // jump OP_RES, // reserved (unused) OP_LEA, // load effective address OP_TRAP // execute trap};// condition flagsenum{ FL_POS = 1 << 0, // P, positive FL_ZRO = 1 << 1, // Z, zero FL_NEG = 1 << 2, // N, negative};// trap codeenum{ TRAP_GETC = 0x20, // get character from keyboard, not echoed onto the terminal TRAP_OUT = 0x21, // output a character TRAP_PUTS = 0x22, // output a word string TRAP_IN = 0x23, // get character from keyboard, echoed onto the terminal TRAP_PUTSP = 0x24, // output a byte string TRAP_HALT = 0x25, // halt the program};enum{ MR_KBSR = 0xFE00, // keyboard status MR_KBDR = 0xFE02, // keyboard data};void mem_write(uint16_t address, uint16_t val){ memory[address] = val;}#if __linux__ || __APPLE__struct termios original_tio;void disable_input_buffering(){ tcgetattr(STDIN_FILENO, &original_tio); struct termios new_tio = original_tio; new_tio.c_lflag &= ~ICANON & ~ECHO; tcsetattr(STDIN_FILENO, TCSANOW, &new_tio);}void restore_input_buffering(){ tcsetattr(STDIN_FILENO, TCSANOW, &original_tio);}uint16_t check_key(){ fd_set readfds; FD_ZERO(&readfds); FD_SET(STDIN_FILENO, &readfds); struct timeval timeout; timeout.tv_sec = 0; timeout.tv_usec = 0; return select(1, &readfds, NULL, NULL, &timeout) != 0;}#elif _MSC_VERHANDLE hStdin = INVALID_HANDLE_VALUE;DWORD fdwMode, fdwOldMode;void disable_input_buffering(){ hStdin = GetStdHandle(STD_INPUT_HANDLE); GetConsoleMode(hStdin, &fdwOldMode); /* save old mode */ fdwMode = fdwOldMode ^ ENABLE_ECHO_INPUT /* no input echo */ ^ ENABLE_LINE_INPUT; /* return when one or more characters are available */ SetConsoleMode(hStdin, fdwMode); /* set new mode */ FlushConsoleInputBuffer(hStdin); /* clear buffer */}void restore_input_buffering(){ SetConsoleMode(hStdin, fdwOldMode);}uint16_t check_key(){ return WaitForSingleObject(hStdin, 1000) == WAIT_OBJECT_0 && _kbhit();}#endifvoid handle_interrupt(int signal){ restore_input_buffering(); printf("/n"); exit(-2);}uint16_t mem_read(uint16_t address){ if (address == MR_KBSR) { if (check_key()) { memory[MR_KBSR] = (1 << 15); memory[MR_KBDR] = getchar(); } else { memory[MR_KBSR] = 0; } } return memory[address];}int16_t sign_extend(int16_t x, int bit_count){ if ((x >> (bit_count - 1)) & 1) { x |= (0xFFFF << bit_count); } return x;}// 取 instr 的最后 count 位， 然后做带符号的扩展int16_t get_instr_last_part(uint16_t instr, uint8_t count){ return sign_extend(instr & ((1U << count) - 1), count);}void update_flags(uint16_t r){ if (reg[r] == 0) { reg[R_COND] = FL_ZRO; } else if (reg[r] >> 15) /* a 1 in the left-most bit indicates negative */ { reg[R_COND] = FL_NEG; } else { reg[R_COND] = FL_POS; }}void do_add_instruction(uint16_t instr){ uint16_t dr = (instr >> 9) & 0x7; // destination register uint16_t sr1 = (instr >> 6) & 0x7; // first source register uint16_t imm_flag = (instr >> 5) & 0x1; // whether we are in immediate mode if (imm_flag) { uint16_t imm5 = get_instr_last_part(instr, 5); reg[dr] = reg[sr1] + imm5; } else { uint16_t sr2 = instr & 0x7; reg[dr] = reg[sr1] + reg[sr2]; } update_flags(dr);}void do_and_instruction(uint16_t instr){ uint16_t dr = (instr >> 9) & 0x7; // destination register uint16_t sr1 = (instr >> 6) & 0x7; // first source register uint16_t imm_flag = (instr >> 5) & 0x1; // whether we are in immediate mode if (imm_flag) { uint16_t imm5 = get_instr_last_part(instr, 5); reg[dr] = reg[sr1] & imm5; } else { uint16_t sr2 = instr & 0x7; reg[dr] = reg[sr1] & reg[sr2]; } update_flags(dr);}void do_br_instruction(uint16_t instr){ uint16_t pc_offset = get_instr_last_part(instr, 9); uint16_t cond_flag = (instr >> 9) & 0x7; if (cond_flag & reg[R_COND]) { reg[R_PC] = reg[R_PC] + pc_offset; }}void do_jmp_instruction(uint16_t instr){ uint16_t base_r = (instr >> 6) & 0x7; reg[R_PC] = reg[base_r];}void do_jsr_instruction(uint16_t instr){ reg[R_R7] = reg[R_PC]; uint16_t flag = (instr >> 11) & 0x1; if (flag) { uint16_t pc_offset = get_instr_last_part(instr, 11); reg[R_PC] = reg[R_PC] + pc_offset; } else { uint16_t base_r = (instr >> 6) & 0x7; reg[R_PC] = base_r; }}void do_ld_instruction(uint16_t instr){ uint16_t dr = (instr >> 9) & 0x7; uint16_t pc_offset = get_instr_last_part(instr, 9); reg[dr] = mem_read(reg[R_PC] + pc_offset); update_flags(dr);}void do_ldi_instruction(uint16_t instr){ uint16_t dr = (instr >> 9) & 0x7; // destination register uint16_t pc_offset = get_instr_last_part(instr, 9); reg[dr] = mem_read(mem_read(reg[R_PC] + pc_offset)); update_flags(dr);}void do_ldr_instruction(uint16_t instr){ uint16_t offset = instr & 0x3F; uint16_t base_r = (instr >> 6) & 0x7; uint16_t dr = (instr >> 9) & 0x7; reg[dr] = mem_read(reg[base_r] + offset); update_flags(dr);}void do_lea_instruction(uint16_t instr){ uint16_t pc_offset = instr & 0x1FF; uint16_t dr = (instr >> 9) & 0x7; reg[dr] = reg[R_PC] + pc_offset; update_flags(dr);}void do_not_instruction(uint16_t instr){ uint16_t dr = (instr >> 9) & 0x7; uint16_t sr = (instr >> 6) & 0x7; reg[dr] = ~(reg[sr]); update_flags(dr);}void do_ret_instruction(uint16_t instr){ reg[R_PC] = reg[R_R7];}void do_res_instruction(uint16_t instr){ abort();}void do_st_instruction(uint16_t instr){ uint16_t offset = get_instr_last_part(instr, 9); uint16_t sr = (instr >> 9) & 0x7; //memory[reg[R_PC] + offset] = reg[sr]; mem_write(reg[R_PC] + offset, reg[sr]);}void do_sti_instruction(uint16_t instr){ uint16_t offset = get_instr_last_part(instr, 9); uint16_t sr = (instr >> 9) & 0x7; //memory[memory[reg[R_PC] + offset]] = reg[sr]; mem_write(mem_read(reg[R_PC] + offset), reg[sr]);}void print_binary(uint16_t n) { char buffer[17]; /* 16 bits, plus room for a /0 */ buffer[16] = '/0'; for (int i = 15; i >= 0; --i) { /* convert bits from the end */ buffer[i] = '0' + (n & 1); /* '0' + bit => '0' or '1' */ n >>= 1; /* make the next bit the 'low bit' */ } /* At this point, `buffer` is `n` turned to a string of binary digits. */ printf("%s/n", buffer);}void do_str_instruction(uint16_t instr){ uint16_t sr = (instr >> 9) & 0x7; uint16_t base_r = (instr >> 6) & 0x7; int16_t offset = get_instr_last_part(instr, 6); memory[reg[base_r] + offset] = reg[sr]; // segfault}void do_puts_trap(){ uint16_t* c = memory + reg[R_R0]; while (*c) { putc((char)*c, stdout); c++; } fflush(stdout);}void do_in_trap(){ printf("Enter a character: "); char c = getchar(); putc(c, stdout); fflush(stdout); reg[R_R0] = (uint16_t)c; update_flags(R_R0); // ??}void do_putsp_trap(){ uint16_t* c = memory + reg[R_R0]; while (*c) { char char1 = (*c) & 0xFF; putc(char1, stdout); char char2 = (*c) >> 8; if (char2) { putc(char2, stdout); c++; } } fflush(stdout);}void do_halt_trap(){ puts("HALT"); fflush(stdout); running = 0;}void do_getc_trap(){ reg[R_R0] = (uint16_t)getchar(); update_flags(R_R0); // ??}void do_output_trap(){ putc((char)reg[R_R0], stdout); fflush(stdout);}void do_trap_instruction(uint16_t instr){ uint16_t trapvec = instr & 0xFF; reg[R_R7] = reg[R_PC]; // 处理 trap switch (trapvec) { case TRAP_GETC: PX_LOGE(" TRAP_GETC"); do_getc_trap(); break; case TRAP_OUT: PX_LOGE(" TRAP_OUT"); do_output_trap(); break; case TRAP_PUTS: PX_LOGE(" TRAP_PUTS"); do_puts_trap(); break; case TRAP_IN: PX_LOGE(" TRAP_IN"); do_in_trap(); break; case TRAP_PUTSP: PX_LOGE(" TRAP_PUTSP"); do_putsp_trap(); break; case TRAP_HALT: PX_LOGE(" TRAP_HALT"); do_halt_trap(); break; } reg[R_PC] = reg[R_R7]; // 由于本函数中的实现， 并没有修改 PC, 因此恢复 PC 看起来有点多余。 但稳妥的做法确实应该是恢复 PC 的。}uint16_t swap16(uint16_t x){ return (x << 8) | (x >> 8);}static inline bool is_little_endian(){ volatile uint32_t data = 0x01234567; return (*((uint8_t*)(&data))) == 0x67;}void read_image_file(FILE* file){ uint16_t origin; // the origin tells us where in memory to place the image fread(&origin, sizeof(origin), 1, file); if (is_little_endian()) { origin = swap16(origin); } // we know the maximum file size so we only need one fread uint16_t max_read = UINT16_MAX - origin; uint16_t* p = memory + origin; size_t read = fread(p, sizeof(uint16_t), max_read, file); // swap to little endian if (is_little_endian()) { while (read-- > 0) { *p = swap16(*p); ++p; } }}int read_image(const char* image_path){ FILE* file = fopen(image_path, "rb"); if (!file) { return 0; } read_image_file(file); fclose(file); return 1;}int main(int argc, char** argv){ if (argc != 2) { fprintf(stderr, "Usage: %s /path/to/image-file/n", argv[0]); exit(1); } if (!read_image(argv[1])) { fprintf(stderr, "failed to load image: %s/n", argv[1]); exit(1); } // 注册键盘监听信号 signal(SIGINT, handle_interrupt); disable_input_buffering(); reg[R_COND] = FL_ZRO; enum { PC_START = 0x3000 }; reg[R_PC] = PC_START; int op_cnt = 0; while (running) { uint16_t instr = mem_read(reg[R_PC]++); // fetch uint16_t op = instr >> 12; op_cnt++; // if (op_cnt == 10) break; // PX_LOGE("op_cnt = %d ", op_cnt); switch (op) { case OP_ADD: PX_LOGE("OP_ADD"); do_add_instruction(instr); break; case OP_AND: PX_LOGE("OP_AND"); do_and_instruction(instr); break; case OP_NOT: PX_LOGE("OP_NOT"); do_not_instruction(instr); break; case OP_BR: PX_LOGE("OP_BR"); do_br_instruction(instr); break; case OP_JMP: PX_LOGE("OP_JMP"); do_jmp_instruction(instr); break; case OP_JSR: PX_LOGE("OP_JSR"); do_jsr_instruction(instr); break; case OP_LD: PX_LOGE("OP_LD"); do_ld_instruction(instr); break; case OP_LDI: PX_LOGE("OP_LDI"); do_ldi_instruction(instr); break; case OP_LDR: PX_LOGE("OP_LDR"); do_ldr_instruction(instr); break; case OP_LEA: PX_LOGE("OP_LEA"); do_lea_instruction(instr); break; case OP_ST: PX_LOGE("OP_ST"); do_st_instruction(instr); break; case OP_STI: PX_LOGE("OP_STI"); do_sti_instruction(instr); break; case OP_STR: PX_LOGE("OP_STR"); do_str_instruction(instr); break; case OP_TRAP: PX_LOGE("OP_TRAP"); do_trap_instruction(instr); break; case OP_RES: case OP_RTI: default: abort(); break; } } restore_input_buffering(); return 0;}关键词：实现,语言,虚拟
