So i was wondering if there is any? I know afd on windows but not sure anything about mac?
And this his how i am using nasam on the following code: nasm a.asm -o a.com -l a.lst
[org 0x100]
mov ax, 5
mov bx, 10
add ax, bx
mov bx, 15
add ax, bx
mov ax, 0x4c00
int 0x21
On windows i know a debugger name afd which help me to step through each statement but not sure how i can do this using gdb.
And neither i am able to execute this .com file, am i supposed to make some other file here?
Why are you writing 16-bit code that makes DOS syscalls? If you want to know how to write asm that's applicable to your OS, take a look the code generated by "gcc -S" on some C code... (Note that code generated this way will have operands reversed, and is meant to be assembled with as instead of nasm)
Further, are you aware what this code is doing? It reads to me like this:
ax = 5
bx = 10
ax += bx
bx = 15
ax += bx
ax = 0x4c00
int 21h
Seems like this code is equivalent to:
mov bx, 15
mov ax, 4c00
int 21h
Which according to what I see here, is exit(0). You didn't need to change bx either...
But. This doesn't even apply to what you were trying to do, because Mac OS X is not MS-DOS, does not know about DOS APIs, cannot run .COM files, etc. I wasn't even aware that it can run 16 bit code. You will want to look at nasm's -f elf option, and you will want to use registers like eax rather than ax.
I've not done assembly programming on OS X, but you could theoretically do something like this:
extern exit
global main
main:
push dword 0
call exit
; This will never get called, but hey...
add esp, 4
xor eax, eax
ret
Then:
nasm -f elf foo.asm -o foo.o
ld -o foo foo.o -lc
Of course this is relying on the C library, which you might not want to do. I've omitted the "full" version because I don't know what the syscall interface looks like on Mac. On many platforms your entry point is the symbol _start and you do syscalls with int 80h or sysenter.
As for debugging... I would also suggest GDB. You can advance by a single instruction with stepi, and the info registers command will dump register state. The disassemble command is also helpful.
Update: Just remembered, I don't think Mac OS X uses ELF... Well.. Much of what I wrote still applies. :-)
Xcode ships with GDB, the GNU Debugger.
Xcode 4 and newer ships with LLDB instead.
As others have said, use GDB, the gnu debugger. In debugging assembly source, I usually find it useful to load a command file that contains something like the following:
display/5i $pc
display/x $eax
display/x $ebx
...
display/5i will display 5 instructions starting with the next to be executed. You can use the stepi command to step execution one instruction at a time. display/x $eax displays the contents of the eax register in hex. You will also likely want to use the x command to examine the contents of memory: x/x $eax, for example, prints the contents of the memory whose address is stored in eax.
These are a few of many commands. Download the GDB manual and skim through it to find other commands you may be interested in using.
IDA Pro does work on the Mac after a fashion (UI still runs on Windows; see an example).
Related
I wrote the following code to check if the 1st number- 'x' is greater than the 2nd number- 'y'. For x>y output should be 1 and for x<=y output should be 0.
section .txt
global _start
global checkGreater
_start:
mov rdi,x
mov rsi,y
call checkGreater
mov rax,60
mov rdi,0
syscall
checkGreater:
mov r8,rdi
mov r9,rsi
cmp r8,r9
jg skip
mov [c],byte '0'
skip:
mov rax,1
mov rdi,1
mov rsi,c
mov rdx,1
syscall
ret
section .data
x db 7
y db 5
c db '1',0
But due to some reasons(of course from my end), the code always gives 0 as the output when executed.
I am using the following commands to run the code on Ubuntu 20.04.1 LTS with nasm 2.14.02-1
nasm -f elf64 fileName.asm
ld -s -o fileName fileName.o
./fileName
Where did I make a mistake?
And how should one debug assembly codes, I looked for printing received arguments in checkGreater, but it turns out that's a disturbing headache itself.
Note: If someone wondering why I didn't directly use x and y in checkGreater, I want to extend the comparison to user inputs, and so wrote code in that way only.
The instructions
mov rdi,x
mov rsi,y
write the address of x into rdi, and of y into rsi. The further code then goes on to compare the addresses, which are always x<y, since x is defined above y.
What you should have written instead is
mov rdi,[x]
mov rsi,[y]
But then you have another problem: x and y variables are 1 byte long, while the destination registers are 8 bytes long. So simply doing the above fix will read extraneous bytes, leading to useless results. The final correction is to either fix the size of the variables (writing dq instead of db), or read them as bytes:
movzx rdi,byte [x]
movzx rsi,byte [y]
As for
And how should one debug assembly codes
The main tool for you is an assembly-level debugger, like EDB on Linux or x64dbg on Windows. But in fact, most debuggers, even the ones intended for languages like C++, are capable of displaying disassembly for the program being debugged. So you can use e.g. GDB, or even a GUI wrapper for it like Qt Creator or Eclipse. Just be sure to switch to machine code mode, or use the appropriate commands like GDB's disassemble, stepi, info registers etc..
Note that you don't have to build EDB or GDB from source (as the links above might suggest): they are likely already packaged in the Linux distribution you use. E.g. on Ubuntu the packages are called edb-debugger and gdb.
I'm new to GNU Debugger. I've been playing around with it, debugging Assembly Files (x86_64 Linux) for a day or so and just a few hours ago I ''discovered'' the TUI interface.
My first attempt using the TUI interface was to see the register changes as I execute each line at a time of a simple Hello World program (in asm). Here is the code of the program
section .data
text db "Hello, World!", 10
len equ $-text
section .text
global _start
_start:
nop
call _printText
mov rax, 60
mov rdi, 0
syscall
_printText:
nop
mov rax, 1
mov rdi, 1
mov rsi, text
mov rdx, len
syscall
ret
After creating the executable file in the terminal of linux I write
$ gdb -q ./hello -tui
Then I created three breakpoints: one right of the _start, another right after _printText and the last just above the mov rax, 60 for the SYS_EXIT.
After this:
1) I run the program.
2) On gdb mode I write layout asm to see the written code.
3) I write layout regs.
4) Finally I use stepi to see how the register change according the the written hello world program.
The thing is that when the RIP register points to the address of ret, corresponding to SYS_EXIT and I hit Enter I get the following message in console
[Inferior 1 (process 2059) exited normally]
/build/gdb-cXfXJ3/gdb-7.11.1/gdb/thread.c:1100: internal-error: finish_thread_st
ate: Assertion `tp' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)
If I type n It appears this (as it says, it quits if I type y):
This is a bug, please report it. For instructions, see:
<http://www.gnu.org/software/gdb/bugs/>.
/build/gdb-cXfXJ3/gdb-7.11.1/gdb/thread.c:1100: internal-error: finish_thread_st
ate: Assertion `tp' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Create a core file of GDB? (y or n)
As I don't know what a core file of GDB (and what is useful for), so I type n and the debugging session closes.
Does anyone know why this is happening and how can be fixed?
By the way, I'm new in Assembly also, so if this occurs because of something wrong in the program I'd also appreciate if anyone can point that out.
I use the same GDB version as you and I always use the TUI features; but I've never had this problem. However, when I use your code the internal GDB error occurs. But if I make one change in your write syscall function, the error does not manifest.
Although you are not calling another function from within a function, I generally create a stack frame by including at least the "push rbp", "mov rbp, rsp", and "leave" instructions in my x86-64 function calls. This may be a band-aide or a work around with respect to the "bug".
_printText:
push rbp
mov rbp, rsp
mov rax, 1
mov rdi, 1
mov rsi, text
mov rdx, len
syscall
leave
ret
Does anyone know why this is happening
It's happening because there is a bug in GDB (more precisely, an assertion that GDB internal variable tp is not NULL has been violated).
and how can be fixed?
You should try to reproduce this with current version of GDB (the bug may have already been fixed), and file a bug report (like the message tells you).
I don't know what a core file of GDB (and what is useful for),
It's only useful to GDB developers.
The book Assembly Language Step by Step provides the following code as a sandbox:
section .data
section .text
global _start
_start:
nop
//insert sandbox code here
nop
Any example that I include in the space for sandbox is creating a segmentation fault. For example, adding this code:
mov ax, 067FEh
mov bx, ax
mov cl, bh
mov ch, bl
Then compiling with:
nasm -f macho sandbox.asm
ld -o sandbox -e _start sandbox.o
creates a seg fault when I run it on my OS/X. Is there a way to get more information about what's causing the segmentation fault?
The problem you have is that you have created a program that runs past the end of the code that you have written.
When your program executes, the loader will end up issuing a jmp to your _start. Your code then runs, but you do not have anything to return to the OS at the end, so it will simply continue running, executing whatever bytes happen to be in RAM after your code.
The simplest fix would be to properly exit the code. For example:
mov eax, 0x1 ; system call number for exit
sub esp, 4 ; OS X system calls needs "extra space" on stack
int 0x80
Since you are not generating any actual output, you would need to step through with a debugger to see what's going on. After compiling you could use lldb to step through.
lldb ./sandbox
image dump sections
Make note of the address listed that is of type code for your executable (not dyld). It will likely be 0x0000000000001fe6. Continuing within lldb:
b s -a 0x0000000000001fe6
run
register read
step
register read
step
register read
At this point you should be past the NOPs and see things changing in registers. Have fun!
Does anyone know of any good tools (I'm looking for IDEs) to write assembly on the Mac. Xcode is a little cumbersome to me.
Also, on the Intel Macs, can I use generic x86 asm? Or is there a modified instruction set? Any information about post Intel.
Also: I know that on windows, asm can run in an emulated environment created by the OS to let the code think it's running on its own dedicated machine. Does OS X provide the same thing?
After installing any version of Xcode targeting Intel-based Macs, you should be able to write assembly code. Xcode is a suite of tools, only one of which is the IDE, so you don't have to use it if you don't want to. (That said, if there are specific things you find clunky, please file a bug at Apple's bug reporter - every bug goes to engineering.) Furthermore, installing Xcode will install both the Netwide Assembler (NASM) and the GNU Assembler (GAS); that will let you use whatever assembly syntax you're most comfortable with.
You'll also want to take a look at the Compiler & Debugging Guides, because those document the calling conventions used for the various architectures that Mac OS X runs on, as well as how the binary format and the loader work. The IA-32 (x86-32) calling conventions in particular may be slightly different from what you're used to.
Another thing to keep in mind is that the system call interface on Mac OS X is different from what you might be used to on DOS/Windows, Linux, or the other BSD flavors. System calls aren't considered a stable API on Mac OS X; instead, you always go through libSystem. That will ensure you're writing code that's portable from one release of the OS to the next.
Finally, keep in mind that Mac OS X runs across a pretty wide array of hardware - everything from the 32-bit Core Single through the high-end quad-core Xeon. By coding in assembly you might not be optimizing as much as you think; what's optimal on one machine may be pessimal on another. Apple regularly measures its compilers and tunes their output with the "-Os" optimization flag to be decent across its line, and there are extensive vector/matrix-processing libraries that you can use to get high performance with hand-tuned CPU-specific implementations.
Going to assembly for fun is great. Going to assembly for speed is not for the faint of heart these days.
As stated before, don't use syscall. You can use standard C library calls though, but be aware that the stack MUST be 16 byte aligned per Apple's IA32 function call ABI.
If you don't align the stack, your program will crash in __dyld_misaligned_stack_error when you make a call into any of the libraries or frameworks.
The following snippet assembles and runs on my system:
; File: hello.asm
; Build: nasm -f macho hello.asm && gcc -o hello hello.o
SECTION .rodata
hello.msg db 'Hello, World!',0x0a,0x00
SECTION .text
extern _printf ; could also use _puts...
GLOBAL _main
; aligns esp to 16 bytes in preparation for calling a C library function
; arg is number of bytes to pad for function arguments, this should be a multiple of 16
; unless you are using push/pop to load args
%macro clib_prolog 1
mov ebx, esp ; remember current esp
and esp, 0xFFFFFFF0 ; align to next 16 byte boundary (could be zero offset!)
sub esp, 12 ; skip ahead 12 so we can store original esp
push ebx ; store esp (16 bytes aligned again)
sub esp, %1 ; pad for arguments (make conditional?)
%endmacro
; arg must match most recent call to clib_prolog
%macro clib_epilog 1
add esp, %1 ; remove arg padding
pop ebx ; get original esp
mov esp, ebx ; restore
%endmacro
_main:
; set up stack frame
push ebp
mov ebp, esp
push ebx
clib_prolog 16
mov dword [esp], hello.msg
call _printf
; can make more clib calls here...
clib_epilog 16
; tear down stack frame
pop ebx
mov esp, ebp
pop ebp
mov eax, 0 ; set return code
ret
Recently I wanted to learn how to compile Intel x86 on Mac OS X:
For nasm:
-o hello.tmp - outfile
-f macho - specify format
Linux - elf or elf64
Mac OSX - macho
For ld:
-arch i386 - specify architecture (32 bit assembly)
-macosx_version_min 10.6 (Mac OSX - complains about default specification)
-no_pie (Mac OSX - removes ld warning)
-e main - specify main symbol name (Mac OSX - default is start)
-o hello.o - outfile
For Shell:
./hello.o - execution
One-liner:
nasm -o hello.tmp -f macho hello.s && ld -arch i386 -macosx_version_min 10.6 -no_pie -e _main -o hello.o hello.tmp && ./hello.o
Let me know if this helps!
I wrote how to do it on my blog here:
http://blog.burrowsapps.com/2013/07/how-to-compile-helloworld-in-intel-x86.html
For a more verbose explanation, I explained on my Github here:
https://github.com/jaredsburrows/Assembly
Running assembly Code on Mac is just 3 steps away from you. It could be done using XCODE but better is to use NASM Command Line Tool.
For My Ease I have already installed Xcode, if you have Xcode installed its good.
But You can do it without XCode as well.
Just Follow:
First Install NASM using Homebrew brew install nasm
convert .asm file into Obj File using this command nasm -f macho64 myFile.asm
Run Obj File to see OutPut using command ld -macosx_version_min 10.7.0 -lSystem -o OutPutFile myFile.o && ./64
Simple Text File named myFile.asm is written below for your convenience.
global start
section .text
start:
mov rax, 0x2000004 ; write
mov rdi, 1 ; stdout
mov rsi, msg
mov rdx, msg.len
syscall
mov rax, 0x2000001 ; exit
mov rdi, 0
syscall
section .data
msg: db "Assalam O Alaikum Dear", 10
.len: equ $ - msg
Also, on the Intel Macs, can I use generic x86 asm? or is there a modified instruction set? Any information about post Intel Mac assembly helps.
It's the same instruction set; it's the same chips.
The features available to use are dependent on your processor. Apple uses the same Intel stuff as everybody else. So yes, generic x86 should be fine (assuming you're not on a PPC :D).
As far as tools go, I think your best bet is a good text editor that 'understands' assembly.
Forget about finding a IDE to write/run/compile assembler on Mac. But, remember mac is UNIX. See http://asm.sourceforge.net/articles/linasm.html. A decent guide (though short) to running assembler via GCC on Linux. You can mimic this. Macs use Intel chips so you want to look at Intel syntax.
Does anyone know of any good tools (I'm looking for IDEs) to write assembly on the Mac. Xcode is a little cumbersome to me.
Also, on the Intel Macs, can I use generic x86 asm? Or is there a modified instruction set? Any information about post Intel.
Also: I know that on windows, asm can run in an emulated environment created by the OS to let the code think it's running on its own dedicated machine. Does OS X provide the same thing?
After installing any version of Xcode targeting Intel-based Macs, you should be able to write assembly code. Xcode is a suite of tools, only one of which is the IDE, so you don't have to use it if you don't want to. (That said, if there are specific things you find clunky, please file a bug at Apple's bug reporter - every bug goes to engineering.) Furthermore, installing Xcode will install both the Netwide Assembler (NASM) and the GNU Assembler (GAS); that will let you use whatever assembly syntax you're most comfortable with.
You'll also want to take a look at the Compiler & Debugging Guides, because those document the calling conventions used for the various architectures that Mac OS X runs on, as well as how the binary format and the loader work. The IA-32 (x86-32) calling conventions in particular may be slightly different from what you're used to.
Another thing to keep in mind is that the system call interface on Mac OS X is different from what you might be used to on DOS/Windows, Linux, or the other BSD flavors. System calls aren't considered a stable API on Mac OS X; instead, you always go through libSystem. That will ensure you're writing code that's portable from one release of the OS to the next.
Finally, keep in mind that Mac OS X runs across a pretty wide array of hardware - everything from the 32-bit Core Single through the high-end quad-core Xeon. By coding in assembly you might not be optimizing as much as you think; what's optimal on one machine may be pessimal on another. Apple regularly measures its compilers and tunes their output with the "-Os" optimization flag to be decent across its line, and there are extensive vector/matrix-processing libraries that you can use to get high performance with hand-tuned CPU-specific implementations.
Going to assembly for fun is great. Going to assembly for speed is not for the faint of heart these days.
As stated before, don't use syscall. You can use standard C library calls though, but be aware that the stack MUST be 16 byte aligned per Apple's IA32 function call ABI.
If you don't align the stack, your program will crash in __dyld_misaligned_stack_error when you make a call into any of the libraries or frameworks.
The following snippet assembles and runs on my system:
; File: hello.asm
; Build: nasm -f macho hello.asm && gcc -o hello hello.o
SECTION .rodata
hello.msg db 'Hello, World!',0x0a,0x00
SECTION .text
extern _printf ; could also use _puts...
GLOBAL _main
; aligns esp to 16 bytes in preparation for calling a C library function
; arg is number of bytes to pad for function arguments, this should be a multiple of 16
; unless you are using push/pop to load args
%macro clib_prolog 1
mov ebx, esp ; remember current esp
and esp, 0xFFFFFFF0 ; align to next 16 byte boundary (could be zero offset!)
sub esp, 12 ; skip ahead 12 so we can store original esp
push ebx ; store esp (16 bytes aligned again)
sub esp, %1 ; pad for arguments (make conditional?)
%endmacro
; arg must match most recent call to clib_prolog
%macro clib_epilog 1
add esp, %1 ; remove arg padding
pop ebx ; get original esp
mov esp, ebx ; restore
%endmacro
_main:
; set up stack frame
push ebp
mov ebp, esp
push ebx
clib_prolog 16
mov dword [esp], hello.msg
call _printf
; can make more clib calls here...
clib_epilog 16
; tear down stack frame
pop ebx
mov esp, ebp
pop ebp
mov eax, 0 ; set return code
ret
Recently I wanted to learn how to compile Intel x86 on Mac OS X:
For nasm:
-o hello.tmp - outfile
-f macho - specify format
Linux - elf or elf64
Mac OSX - macho
For ld:
-arch i386 - specify architecture (32 bit assembly)
-macosx_version_min 10.6 (Mac OSX - complains about default specification)
-no_pie (Mac OSX - removes ld warning)
-e main - specify main symbol name (Mac OSX - default is start)
-o hello.o - outfile
For Shell:
./hello.o - execution
One-liner:
nasm -o hello.tmp -f macho hello.s && ld -arch i386 -macosx_version_min 10.6 -no_pie -e _main -o hello.o hello.tmp && ./hello.o
Let me know if this helps!
I wrote how to do it on my blog here:
http://blog.burrowsapps.com/2013/07/how-to-compile-helloworld-in-intel-x86.html
For a more verbose explanation, I explained on my Github here:
https://github.com/jaredsburrows/Assembly
Running assembly Code on Mac is just 3 steps away from you. It could be done using XCODE but better is to use NASM Command Line Tool.
For My Ease I have already installed Xcode, if you have Xcode installed its good.
But You can do it without XCode as well.
Just Follow:
First Install NASM using Homebrew brew install nasm
convert .asm file into Obj File using this command nasm -f macho64 myFile.asm
Run Obj File to see OutPut using command ld -macosx_version_min 10.7.0 -lSystem -o OutPutFile myFile.o && ./64
Simple Text File named myFile.asm is written below for your convenience.
global start
section .text
start:
mov rax, 0x2000004 ; write
mov rdi, 1 ; stdout
mov rsi, msg
mov rdx, msg.len
syscall
mov rax, 0x2000001 ; exit
mov rdi, 0
syscall
section .data
msg: db "Assalam O Alaikum Dear", 10
.len: equ $ - msg
Also, on the Intel Macs, can I use generic x86 asm? or is there a modified instruction set? Any information about post Intel Mac assembly helps.
It's the same instruction set; it's the same chips.
The features available to use are dependent on your processor. Apple uses the same Intel stuff as everybody else. So yes, generic x86 should be fine (assuming you're not on a PPC :D).
As far as tools go, I think your best bet is a good text editor that 'understands' assembly.
Forget about finding a IDE to write/run/compile assembler on Mac. But, remember mac is UNIX. See http://asm.sourceforge.net/articles/linasm.html. A decent guide (though short) to running assembler via GCC on Linux. You can mimic this. Macs use Intel chips so you want to look at Intel syntax.