https://github.com/akkartik/mu/blob/main/linux/104new-segment.subx
 1 # Create a new segment (pool of memory for allocating chunks from) in the form
 2 # of an *allocation descriptor* that can be passed to the memory allocator
 3 # (defined in a later layer).
 4 #
 5 # Currently an allocation descriptor consists of just the bounds of the pool of
 6 # available memory:
 7 #
 8 #   curr: address
 9 #   end: address
10 #
11 # This isn't enough information to reclaim individual allocations. We can't
12 # support arbitrary reclamation yet.
13 
14 == code
15 #   instruction                     effective address                                                   register    displacement    immediate
16 # . op          subop               mod             rm32          base        index         scale       r32
17 # . 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes
18 
19 #? Entry:   # manual test
20 #?     # var ad/ecx: allocation-descriptor
21 #?     68/push  0/imm32/limit
22 #?     68/push  0/imm32/curr
23 #?     89/copy                         3/mod/direct    1/rm32/ecx    .           .             .           4/r32/esp   .               .                 # copy esp to ecx
24 #?     # new-segment(0x1000, ad)
25 #?     # . . push args
26 #?     51/push-ecx
27 #?     68/push  0x1000/imm32
28 #?     # . . call
29 #?     e8/call  new-segment/disp32
30 #?     # . . discard args
31 #?     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
32 #?     # var eax: (addr _) = ad->curr
33 #?     8b/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy *ecx to eax
34 #?     # write to *eax to check that we have access to the newly-allocated segment
35 #?     c7          0/subop/copy        0/mod/direct    0/rm32/eax    .           .             .           .           .               0x34/imm32        # copy to *eax
36 #?     # syscall_exit(eax)
37 #?     89/copy                         3/mod/direct    3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to ebx
38 #?     e8/call  syscall_exit/disp32
39 
40 new-segment:  # len: int, ad: (addr allocation-descriptor)
41     # . prologue
42     55/push-ebp
43     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
44     # . save registers
45     50/push-eax
46     53/push-ebx
47     # copy len to _mmap-new-segment->len
48     8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   8/disp8         .                 # copy *(ebp+8) to eax
49     89/copy                         0/mod/indirect  5/rm32/.disp32            .             .           0/r32/eax   $_mmap-new-segment:len/disp32     # copy eax to *$_mmap-new-segment:len
50     # mmap(_mmap-new-segment)
51     bb/copy-to-ebx  _mmap-new-segment/imm32
52     e8/call  syscall_mmap/disp32
53     # copy {eax, eax+len} to *ad
54     # . ebx = ad
55     8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           3/r32/ebx   0xc/disp8       .                 # copy *(ebp+12) to ebx
56     # . ad->curr = eax
57     89/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to *ebx
58     # . ad->end = eax+len
59     03/add                          1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   8/disp8         .                 # add *(ebp+8) to eax
60     89/copy                         1/mod/*+disp8   3/rm32/ebx    .           .             .           0/r32/eax   4/disp8         .                 # copy eax to *(ebx+4)
61 $new-segment:end:
62     # . restore registers
63     5b/pop-to-ebx
64     58/pop-to-eax
65     # . epilogue
66     89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
67     5d/pop-to-ebp
68     c3/return
69 
70 == data
71 
72 # various constants used here were found in the Linux sources (search for file mman-common.h)
73 _mmap-new-segment:  # mmap_arg_struct
74     # addr
75     0/imm32
76 $_mmap-new-segment:len:
77     # len
78     0/imm32
79     # protection flags
80     3/imm32  # PROT_READ | PROT_WRITE
81     # sharing flags
82     0x22/imm32  # MAP_PRIVATE | MAP_ANONYMOUS
83     # fd
84     -1/imm32  # since MAP_ANONYMOUS is specified
85     # offset
86     0/imm32  # since MAP_ANONYMOUS is specified
87 
88 # . . vim:nowrap:textwidth=0