1 :(before "End Initialize Op Names")
  2 put_new(Name, "cd", "software interrupt (int)");
  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     Reg[EAX].i = read(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
 26     trace(91, "run") << "result: " << Reg[EAX].i << end();
 27     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 28     break;
 29   case 4:
 30     trace(91, "run") << "write: " << Reg[EBX].u << ' ' << Reg[ECX].u << ' ' << Reg[EDX].u << end();
 31     trace(91, "run") << Reg[ECX].u << " => " << mem_addr_string(Reg[ECX].u, Reg[EDX].u) << end();
 32     Reg[EAX].i = write(/*file descriptor*/Reg[EBX].u, /*memory buffer*/mem_addr_u8(Reg[ECX].u), /*size*/Reg[EDX].u);
 33     trace(91, "run") << "result: " << Reg[EAX].i << end();
 34     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 35     break;
 36   case 5: {
 37     check_flags(ECX);
 38     check_mode(EDX);
 39     trace(91, "run") << "open: " << Reg[EBX].u << ' ' << Reg[ECX].u << end();
 40     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
 41     Reg[EAX].i = open(/*filename*/mem_addr_kernel_string(Reg[EBX].u), /*flags*/Reg[ECX].u, /*mode*/0640);
 42     trace(91, "run") << "result: " << Reg[EAX].i << end();
 43     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 44     break;
 45   }
 46   case 6:
 47     trace(91, "run") << "close: " << Reg[EBX].u << end();
 48     Reg[EAX].i = close(/*file descriptor*/Reg[EBX].u);
 49     trace(91, "run") << "result: " << Reg[EAX].i << end();
 50     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 51     break;
 52   case 8:
 53     check_mode(ECX);
 54     trace(91, "run") << "creat: " << Reg[EBX].u << end();
 55     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
 56     Reg[EAX].i = creat(/*filename*/mem_addr_kernel_string(Reg[EBX].u), /*mode*/0640);
 57     trace(91, "run") << "result: " << Reg[EAX].i << end();
 58     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 59     break;
 60   case 10:
 61     trace(91, "run") << "unlink: " << Reg[EBX].u << end();
 62     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
 63     Reg[EAX].i = unlink(/*filename*/mem_addr_kernel_string(Reg[EBX].u));
 64     trace(91, "run") << "result: " << Reg[EAX].i << end();
 65     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 66     break;
 67   case 38:
 68     trace(91, "run") << "rename: " << Reg[EBX].u << " -> " << Reg[ECX].u << end();
 69     trace(91, "run") << Reg[EBX].u << " => " << mem_addr_kernel_string(Reg[EBX].u) << end();
 70     trace(91, "run") << Reg[ECX].u << " => " << mem_addr_kernel_string(Reg[ECX].u) << end();
 71     Reg[EAX].i = rename(/*old filename*/mem_addr_kernel_string(Reg[EBX].u), /*new filename*/mem_addr_kernel_string(Reg[ECX].u));
 72     trace(91, "run") << "result: " << Reg[EAX].i << end();
 73     if (Reg[EAX].i == -1) raise << strerror(errno) << '\n' << end();
 74     break;
 75   case 45:  // brk: modify size of data segment
 76     trace(91, "run") << "grow data segment to " << Reg[EBX].u << end();
 77     grow_data_segment(/*new end address*/Reg[EBX].u);
 78     break;
 79   case 90:  // mmap: allocate memory outside existing segment allocations
 80     trace(91, "run") << "mmap: allocate new segment" << end();
 81     // Ignore most arguments for now: address hint, protection flags, sharing flags, fd, offset.
 82     // We only support anonymous maps.
 83     Reg[EAX].u = new_segment(/*length*/read_mem_u32(Reg[EBX].u+0x4));
 84     trace(91, "run") << "result: " << Reg[EAX].u << end();
 85     break;
 86   default:
 87     raise << HEXWORD << EIP << ": unimplemented syscall " << Reg[EAX].u << '\n' << end();
 88   }
 89 }
 90 
 91 // SubX is oblivious to file permissions, directories, symbolic links, terminals, and much else besides.
 92 // Also ignoring any concurrency considerations for now.
 93 void check_flags(int reg) {
 94   uint32_t flags = Reg[reg].u;
 95   if (flags != ((flags & O_RDONLY) | (flags & O_WRONLY))) {
 96     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";
 97     exit(1);
 98   }
 99   if ((flags & O_RDONLY) && (flags & O_WRONLY)) {
100     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";
101     exit(1);
102   }
103 }
104 
105 void check_mode(int reg) {
106   if (Reg[reg].u != 0600) {
107     cerr << HEXWORD << EIP << ": SubX is oblivious to file permissions; register " << reg << " must be 0.\n";
108     exit(1);
109   }
110 }
111 
112 :(before "End Globals")
113 uint32_t Next_segment = 0xb0000000;  // 0xc0000000 and up is reserved for Linux kernel
114 const uint32_t SPACE_FOR_SEGMENT = 0x01000000;
115 :(code)
116 uint32_t new_segment(uint32_t lengt