Started working on decoding 8086 instruction homework

8086_assembly_01/.gitignore

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+dasm
+listing_0037_single_register_mov
+listing_0038_many_register_mov

8086_assembly_01/Makefile

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+all:
+	clang++ -g dasm.cpp -o dasm

8086_assembly_01/dasm.cpp

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+#include <bits/types/FILE.h>
+#include <cstdint>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+
+// Bit patterns for the 8086 OPCODES
+enum class OPCODES {
+  MOV = 0x88,
+};
+
+enum class OPMASKS {
+  MOV = 0xfc,
+};
+
+enum class FLAGS {
+  WORD = 0x01,
+  REG_DEST = 0x02,
+};
+
+enum class MODE {
+  MEM = 0x00,
+  MEM8 = 0x40,
+  MEM16 = 0x80,
+  REG = 0xc0,
+};
+
+uint16_t decode_mode(uint16_t instruction);
+void decode_register(uint16_t instruction, bool word, char *dest);
+
+int main(int argc, char *argv[]) {
+  if (argc < 2) {
+    printf("Please provide a file to disassemble\n");
+    return 1;
+  }
+
+  const char *filename = argv[1];
+
+  FILE *fp = fopen(filename, "rb");
+
+  if (fp) {
+    uint16_t inst = 0;
+    const char *op = "";
+
+    char out_filename[4096] = {0};
+    sprintf(out_filename, "%s_out.asm", filename);
+
+    FILE *out = fopen(out_filename, "w");
+
+    if (out) {
+      fprintf(out, "; Disassembled by DASM\n\nbits 16\n\n");
+
+      while (fread(&inst, sizeof(inst), 1, fp)) {
+        if ((inst & (uint16_t)OPMASKS::MOV) == (uint16_t)OPCODES::MOV) {
+          op = "mov";
+
+          bool reg_dest =
+              (inst & (uint16_t)FLAGS::REG_DEST) == (uint16_t)FLAGS::REG_DEST;
+          bool word = (inst & (uint16_t)FLAGS::WORD) == (uint16_t)FLAGS::WORD;
+
+          // NOTE: Using right shift will only work on little-endian CPUs
+          uint16_t operands_info = inst >> 8;
+
+          if (decode_mode(operands_info) == (uint16_t)MODE::REG) {
+            char rm[3] = {0};
+            char reg[3] = {0};
+
+            decode_register(operands_info, word, rm);
+            decode_register(operands_info >> 3, word, reg);
+
+            fprintf(out, "%s %s, %s\n", op, reg_dest ? reg : rm,
+                    reg_dest ? rm : reg);
+          }
+        } else {
+          printf("It's not a mov operation\n");
+        }
+      }
+
+      fclose(out);
+    } else {
+      printf("Failed to open output file\n");
+    }
+
+    fclose(fp);
+  } else {
+    printf("Failed to open the selected file\n");
+  }
+
+  return 0;
+}
+
+uint16_t decode_mode(uint16_t instruction) {
+  uint16_t mode_mask = 0xc0;
+
+  return instruction & mode_mask;
+}
+
+void decode_register(uint16_t instruction, bool word, char *dest) {
+  uint16_t reg_mask = 0x07;
+
+  switch (instruction & reg_mask) {
+  case 0x00:
+    if (word) {
+      strcpy(dest, "ax");
+    } else {
+      strcpy(dest, "al");
+    }
+    break;
+  case 0x01:
+    if (word) {
+      strcpy(dest, "cx");
+    } else {
+      strcpy(dest, "cl");
+    }
+    break;
+  case 0x02:
+    if (word) {
+      strcpy(dest, "dx");
+    } else {
+      strcpy(dest, "dl");
+    }
+    break;
+  case 0x03:
+    if (word) {
+      strcpy(dest, "bx");
+    } else {
+      strcpy(dest, "bl");
+    }
+    break;
+  case 0x04:
+    if (word) {
+      strcpy(dest, "sp");
+    } else {
+      strcpy(dest, "ah");
+    }
+    break;
+  case 0x05:
+    if (word) {
+      strcpy(dest, "bp");
+    } else {
+      strcpy(dest, "ch");
+    }
+    break;
+  case 0x06:
+    if (word) {
+      strcpy(dest, "si");
+    } else {
+      strcpy(dest, "dh");
+    }
+    break;
+  case 0x07:
+    if (word) {
+      strcpy(dest, "di");
+    } else {
+      strcpy(dest, "bh");
+    }
+    break;
+  }
+}

8086_assembly_01/listing_0037_single_register_mov.asm

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+; ========================================================================
+;
+; (C) Copyright 2023 by Molly Rocket, Inc., All Rights Reserved.
+;
+; This software is provided 'as-is', without any express or implied
+; warranty. In no event will the authors be held liable for any damages
+; arising from the use of this software.
+;
+; Please see https://computerenhance.com for further information
+;
+; ======================================================================== */
+
+; ========================================================================
+; LISTING 37
+; ========================================================================
+
+bits 16
+
+mov cx, bx

8086_assembly_01/listing_0037_single_register_mov_out.asm

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+; Disassembled by DASM
+
+bits 16
+
+mov cx, bx

8086_assembly_01/listing_0038_many_register_mov.asm

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+; ========================================================================
+;
+; (C) Copyright 2023 by Molly Rocket, Inc., All Rights Reserved.
+;
+; This software is provided 'as-is', without any express or implied
+; warranty. In no event will the authors be held liable for any damages
+; arising from the use of this software.
+;
+; Please see https://computerenhance.com for further information
+;
+; ======================================================================== */
+
+; ========================================================================
+; LISTING 38
+; ========================================================================
+
+bits 16
+
+mov cx, bx
+mov ch, ah
+mov dx, bx
+mov si, bx
+mov bx, di
+mov al, cl
+mov ch, ch
+mov bx, ax
+mov bx, si
+mov sp, di
+mov bp, ax

8086_assembly_01/listing_0038_many_register_mov_out.asm

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+; Disassembled by DASM
+
+bits 16
+
+mov cx, bx
+mov ch, ah
+mov dx, bx
+mov si, bx
+mov bx, di
+mov al, cl
+mov ch, ch
+mov bx, ax
+mov bx, si
+mov sp, di
+mov bp, ax