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| author | Kartik Agaram <vc@akkartik.com> | 2018-07-16 11:03:08 -0700 |
|---|---|---|
| committer | Kartik Agaram <vc@akkartik.com> | 2018-07-16 11:05:19 -0700 |
| commit | 0c57224ccff9727fb699994a4c3d5f5c6d6d1881 (patch) | |
| tree | 50608e44d326f83b12381b41bf0ebdb66f8563f8 /subx | |
| parent | 66050b796e436735c74504b90f915dd6bc66b8b0 (diff) | |
| download | mu-0c57224ccff9727fb699994a4c3d5f5c6d6d1881.tar.gz | |
4356 - subx: first program with a data segment
We read() a character from stdin and write() it out to stdout, saving it to a global variable in between. ELF binaries are inefficient; you can ask for a low alignment, but the kernel may not be able to handle it. If you set up a high alignment then you end up wasting an increasing amount of space in each segment because of the constraint that the offset bear some relationship with the loaded address.
Diffstat (limited to 'subx')
| -rw-r--r-- | subx/021translate.cc | 6 | ||||
| -rwxr-xr-x | subx/ex1 | bin | 96 -> 96 bytes | |||
| -rwxr-xr-x | subx/ex2 | bin | 102 -> 102 bytes | |||
| -rwxr-xr-x | subx/ex3 | bin | 119 -> 119 bytes | |||
| -rwxr-xr-x | subx/ex4 | bin | 113 -> 171 bytes | |||
| -rw-r--r-- | subx/ex4.subx | 27 |
6 files changed, 24 insertions, 9 deletions
diff --git a/subx/021translate.cc b/subx/021translate.cc index 4cecb030..2aa4efaa 100644 --- a/subx/021translate.cc +++ b/subx/021translate.cc @@ -88,9 +88,9 @@ void dump_elf_header(ostream& out, const program& p) { // p_offset emit(p_offset); // p_vaddr - emit(e_entry); + emit(p.segments.at(i).start); // p_paddr - emit(e_entry); + emit(p.segments.at(i).start); // p_filesz uint32_t size = size_of(p.segments.at(i)); assert(size < SEGMENT_SIZE); @@ -101,7 +101,7 @@ void dump_elf_header(ostream& out, const program& p) { uint32_t p_flags = (i == 0) ? /*r-x*/0x5 : /*rw-*/0x6; // convention: only first segment is code emit(p_flags); // p_align - uint32_t p_align = 0x4; + uint32_t p_align = 0x1000; // smaller alignments may not be respected by the kernel emit(p_align); // prepare for next segment diff --git a/subx/ex1 b/subx/ex1 index c4114dfd..f3c9730d 100755 --- a/subx/ex1 +++ b/subx/ex1 Binary files differdiff --git a/subx/ex2 b/subx/ex2 index 38520d38..3bbd979b 100755 --- a/subx/ex2 +++ b/subx/ex2 Binary files differdiff --git a/subx/ex3 b/subx/ex3 index 9c1f9a92..aa209b92 100755 --- a/subx/ex3 +++ b/subx/ex3 Binary files differdiff --git a/subx/ex4 b/subx/ex4 index 21f7118b..15529a9f 100755 --- a/subx/ex4 +++ b/subx/ex4 Binary files differdiff --git a/subx/ex4.subx b/subx/ex4.subx index 378b1a35..f76f864e 100644 --- a/subx/ex4.subx +++ b/subx/ex4.subx @@ -1,15 +1,15 @@ ## read a character from stdin -== 0x08048054 # code segment, after leaving room for ELF header +== 0x08048074 # code segment, after leaving room for ELF header and segment headers # opcode ModR/M SIB displacement immediate # instruction mod, reg, Reg/Mem bits scale, index, base # 1-3 bytes 0/1 byte 0/1 byte 0/1/2/4 bytes 0/1/2/4 bytes -# 0: e_entry = 0x08048054 - ## read(1, x, 1) - # fd = 1 - bb 1/imm32 # copy 1 to EBX + + ## read(stdin, x, 1) + # fd = 0 (stdin) + bb 0/imm32 # copy 0 to EBX # set location to write to - b9 00 90 04 08 # copy 0x08040000 to ECX + b9 a7 90 04 08 # copy 0x080490a7 to ECX # size = 1 character ba 1/imm32 # copy 1 to EDX # syscall = read @@ -17,8 +17,23 @@ # call cd 128/imm8 # int 80h + ## write(stdout, x, 1) + # fd = 1 (stdout) + bb 1/imm32 # copy 1 to EBX + # set location to write to + b9 a7 90 04 08 # copy 0x080490a7 to ECX + # size = 1 character + ba 1/imm32 # copy 1 to EDX + # syscall = write + b8 4/imm32 # copy 3 to EAX + # call + cd 128/imm8 # int 80h + ## exit(EBX) b8 1/imm32 # copy 1 to EAX cd 128/imm8 # int 80h +== 0x080490a7 +00 00 00 00 # space for read() to write to + # vim:ft=subx:nowrap |