5151 - use mmap everywhere we need a heap

All tests passing now. Things are very explicit; before a program can `allocate`
memory, it has to first obtain a segment from the OS using `new-segment`.

1 files changed, 12 insertions, 5 deletions

diff --git a/subx/012elf.cc b/subx/012elf.cc
index 11e0e4bc..d0a3fbd2 100644
--- a/subx/012elf.cc
+++ b/subx/012elf.cc
@@ -134,13 +134,20 @@ void load_segment_from_program_header(uint8_t* elf_contents, int segment_index,
 //   code:  0x09000000 -> 0x09ffffff (specified in ELF binary)
 //   data:  0x0a000000 -> 0x0affffff (specified in ELF binary)
 //   --- heap gets mmap'd somewhere here ---
-//   stack: 0xbdffffff -> 0xbd000000 (downward; not in ELF binary)
-//   argv hack: 0xbf000000 -> 0xc0000000 (not in ELF binary)
+//   stack: 0x7dffffff -> 0x7d000000 (downward; not in ELF binary)
+//   argv hack: 0x7f000000 -> 0x7fffffff (not in ELF binary)
+//
+// For now we avoid addresses with the most significant bit set; SubX doesn't
+// support unsigned comparison yet (https://github.com/akkartik/mu/issues/30)
+// Once we do, we can go up to 0xc0000000; higher addresses are reserved for
+// the Linux kernel.
 const int CODE_SEGMENT      = 0x09000000;
 const int DATA_SEGMENT =      0x0a000000;
-const int STACK_SEGMENT     = 0xbd000000;
-const int AFTER_STACK       = 0xbe000000;
-const int ARGV_DATA_SEGMENT = 0xbf000000;
+const int STACK_SEGMENT     = 0x7d000000;
+const int AFTER_STACK       = 0x7e000000;
+const int ARGV_DATA_SEGMENT = 0x7f000000;
+// When updating the above memory map, don't forget to update `mmap`'s
+// implementation in the 'syscalls' layer.
 :(before "End Dump Info for Instruction")
 //? dump_stack();  // slow
 :(code)