1 :(before "End Initialize Op Names(name)")
  2 put(name, "cd", "software interrupt");
  3 
  4 :(before "End Single-Byte Opcodes")
  5 case 0xcd: {  // int imm8 (software interrupt)
  6   trace(90, "run") << "syscall" << end();
  7   uint8_t code = next();
  8   if (code != 0x80) {
  9     raise << "Unimplemented interrupt code " << HEXBYTE << code << '\n' << end();
 10     raise << "  Only `int 80h` supported for now.\n" << end();
 11     break;
 12   }
 13   process_int80();
 14   break;
 15 }
 16 
 17 :(code)
 18 void process_int80() {
 19   switch (Reg[EAX].u) {
 20   case 1:
 21     exit(/*exit code*/Reg[EBX].u);
 22     break;
 23   case 3:
 24     trace(91, "run") << "read: " << Reg[EBX].u << ' ' << Reg[ECX].u << ' ' << Reg[EDX].u << end();
 25     trace(91, "run") << Reg[ECX].u << " => " << mem_addr_string(Reg[ECX].u) << end();
 26     Reg[EAX].i = read(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
 27     trace(91, "run") << "result: " << Reg[EAX].i << end();
 28     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 29     break;
 30   case 4:
 31     trace(91, "run") << "write: " << Reg[EBX].u << ' ' << Reg[ECX].u << ' ' << Reg[EDX].u << end();
 32     trace(91, "run") << Reg[ECX].u << " => " << mem_addr_string(Reg[ECX].u) << end();
 33     Reg[EAX].i = write(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
 34     trace(91, "run") << "result: " << Reg[EAX].i << end();
 35     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 36     break;
 37   case 5: {
 38     check_flags(ECX);
 39     check_mode(EDX);
 40     trace(91, "run") << "open: " << Reg[EBX].u << ' ' << Reg[ECX].u << end();
 41     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_string(Reg[EBX].u) << end();
 42     Reg[EAX].i = open(/*filename*/mem_addr_string(Reg[EBX].u), /*flags*/Reg[ECX].u, /*mode*/0640);
 43     trace(91, "run") << "result: " << Reg[EAX].i << end();
 44     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 45     break;
 46   }
 47   case 6:
 48     trace(91, "run") << "close: " << Reg[EBX].u << end();
 49     Reg[EAX].i = close(/*file descriptor*/Reg[EBX].u);
 50     trace(91, "run") << "result: " << Reg[EAX].i << end();
 51     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 52     break;
 53   case 8:
 54     check_mode(ECX);
 55     trace(91, "run") << "creat: " << Reg[EBX].u << end();
 56     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_string(Reg[EBX].u) << end();
 57     Reg[EAX].i = creat(/*filename*/mem_addr_string(Reg[EBX].u), /*mode*/0640);
 58     trace(91, "run") << "result: " << Reg[EAX].i << end();
 59     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 60     break;
 61   case 10:
 62     trace(91, "run") << "unlink: " << Reg[EBX].u << end();
 63     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_string(Reg[EBX].u) << end();
 64     Reg[EAX].i = unlink(/*filename*/mem_addr_string(Reg[EBX].u));
 65     trace(91, "run") << "result: " << Reg[EAX].i << end();
 66     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 67     break;
 68   case 38:
 69     trace(91, "run") << "rename: " << Reg[EBX].u << " -> " << Reg[ECX].u << end();
 70     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_string(Reg[EBX].u) << end();
 71     trace(91, "run") << Reg[ECX].u << " => " << mem_addr_string(Reg[ECX].u) << end();
 72     Reg[EAX].i = rename(/*old filename*/mem_addr_string(Reg[EBX].u), /*new filename*/mem_addr_string(Reg[ECX].u));
 73     trace(91, "run") << "result: " << Reg[EAX].i << end();
 74     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 75     break;
 76   case 45:  // brk: modify size of data segment
 77     trace(91, "run") << "grow data segment to " << Reg[EBX].u << end();
 78     resize_mem(/*new end address*/Reg[EBX].u);
 79     break;
 80   default:
 81     raise << HEXWORD << EIP << ": unimplemented syscall " << Reg[EAX].u << '\n' << end();
 82   }
 83 }
 84 
 85 // SubX is oblivious to file permissions, directories, symbolic links, terminals, and much else besides.
 86 // Also ignoring any concurrency considerations for now.
 87 void check_flags(int reg) {
 88   uint32_t flags = Reg[reg].u;
 89   if (flags != ((flags & O_RDONLY) | (flags & O_WRONLY))) {
 90     cerr << HEXWORD << EIP << ": most POSIX flags to the open() syscall are not supported. Just O_RDONLY and O_WRONLY for now. Zero concurrent access support.\n";
 91     exit(1);
 92   }
 93   if ((flags & O_RDONLY) && (flags & O_WRONLY)) {
 94     cerr << HEXWORD << EIP << ": can't open a file for both reading and writing at once. See http://man7.org/linux/man-pages/man2/open.2.html.\n";
 95     exit(1);
 96   }
 97 }
 98 
 99 void check_mode(int reg) {
100   if (Reg[reg].u != 0600) {
101     cerr << HEXWORD << EIP << ": SubX is oblivious to file permissions; register " << reg << " must be 0.\n";
102     exit(1);
103   }
104 }
105 
106 void resize_mem(uint32_t new_end_address) {
107   if (new_end_address < Mem_offset) {
108     raise << HEXWORD << EIP << ": can't shrink data segment to before code segment\n";
109     return;
110   }
111   int32_t new_size = new_end_address - Mem_offset;
112   if (new_size < SIZE(Mem)) {
113     raise << HEXWORD << EIP << ": shrinking data segment is not supported.\n" << end();
114     return;
115   }
116   Mem.resize(new_size);  // will throw exception on failure
117 }