Looked a lot online for an answer to this question and found nothing.
My Question Is “How is it possible to access specific address in Assembler “
I’m asking for the actual syntax in Turbo Assembler seen things like
Mov ax , [value]
And
Move es:[bx] , value
And I’m very confused .
What the [value] And :[value] Syntax even mean in Turbo Assembler and how would I access a specific address like B4h for example ?
The “ [ Var ] “ Syntax in Turbo Assembler is simply a way to refer to the value the Var is pointing at in the case Var Holds 3Ah for example this Syntax will refer to the value of the memory address 3Ah .
Related
I'm currently trying to understand the output of the gcc Vectorizer.
I compiled my program using -O2 -ftree-vectorize -fopt-info-vec-all and gcc 8.2.0.
However, I do not understand, what is meant by some of the output messages, and cannot seem to find explanations on the internet.
What is meant by PHI in the following examples?
test.c:14: note: Analyze phi: i_53 = PHI <i_18(7), 0(5)>
test.c:14: note: Access function of PHI: {1024, +, 4294967295}_2
And what is the problem here?
test.c:5: note: not vectorized: not enough data-refs in basic block.
Any help is greatly appreciated.
(I'm not looking for help in solving the issues atm, just trying to understand what they are in the first place)
As to your first question, Phi or 𝛷 functions are a concept in compiler design. At this stage it appears the compiler is expressing your program in static single assignment form, in which every variable can only be written once, and 𝛷 functions are used to select values from among different variables which may not all exist at a given point in a program.
See https://gcc.gnu.org/onlinedocs/gccint/SSA.html for a gcc-specific description.
I don't know the answer to your second question.
MOV is probably the first instruction everyone learns while learning ASM.
Just now I encountered a book Assembly Language Programming in GNU/Linux for IA32 Architectures By Rajat Moona which says: (broken link removed)
But I learnt that it is MOV dest, src. Its like "Load dest with src". Even Wiki says the same.
I'm not saying that the author is wrong. I know that he is right. But what am I missing here?
btw.. he is using GCC's as to assemble these instructions. But that shouldn't change the instruction syntax right?
mov dest, src is called Intel syntax. (e.g. mov eax, 123)
mov src, dest is called AT&T syntax. (e.g. mov $123, %eax)
UNIX assemblers including the GNU assembler uses AT&T syntax, all other x86 assemblers I know of uses Intel syntax. You can read up on the differences on wikipedia.
Yes, as/gas use AT&T syntax that uses the order src,dest. MASM, TASM, NASM, etc. all use the order 'dest, src". As it happens, AT&T syntax doesn't fit very well with Intel processors, and (at least IMO) is a nearly unreadable mess. E.g. movzx comes out particularly bad.
There are two distinct types of assembly language syntax - Intel and AT&T syntax.
You can find a comparison of both on Wikipedia's assembly language page.
Chances are your book uses the AT&T syntax, where the source operand comes before the destination.
As already mentioned in the answer by Jerry Coffin, the Intel syntax fits better with the encoding of instructions for the x86 architecture. As a comment in my debugger's disassembler states, "the operands appear in the instruction in the same order as they appear in the disassembly output". For example, consider this instruction:
-a
1772:0100 test word [AA55], 1234
1772:0106
-u 100 l 1
1772:0100 F70655AA3412 test word [AA55], 1234
-
As you can read in the opcode hexdump, the instruction opcode 0F7h is first, then the ModR/M byte 06h, then the little-endian offset word 0AA55h, and then finally the immediate word 1234h. The Intel syntax matches that order in the assembly source. In the AT&T syntax this would look like testw $0x1234, (0xAA55) which swaps the order compared to the encoding.
Another example that obeys the Intel syntax order is comparison conditions. For example, consider this sequence:
cmp ax, 26
jae .label
This will jump to .label if ax is above-or-equal-to 26 (in unsigned comparison). This mnemonic is only true of the cmp dest, src operand order, which sets flags as for dest -= src.
On Matt Godbolt's Compiler Explorer website, you can compile code using various pre-installed compilers. When using PowerPC gcc 4.8 the registers cannot be distinguished from immediates (for example addi 11,31,16).
However, when the -mregnames option is used, all registers are marked with %r followed by the register index. How do I omit just the % sign to get r1 instead of %r1?
For example, void nop () {} with gcc4.8 PowerPC -O0 -mregnames:
nop():
stwu %r1,-16(%r1)
stw %r31,12(%r1)
mr %r31,%r1
addi %r11,%r31,16
lwz %r31,-4(%r11)
mr %r1,%r11
blr
When targeting PowerPC, you basically have two options for the syntax of assembly listings:
You can either use the IBM syntax (common on IBM assemblers), where the registers do not use any type of special prefix: they are just referred to with numbers. Yes, this makes it difficult to distinguish them from immediates.
Or, you can use Gnu/AT&T syntax, which always prefixes registers with % symbols (and an r, in this case). This not only makes it easier to distinguish between registers and immediates, but it also makes it possible to distinguish between integer registers (%r?) and floating-point registers (%f?).
There is no intermediate option, where you get the r (or f) prefix, but no leading %. If you need this, you can do like Jester suggested and post-process the output, using the regular expression %r[0-9]+ for matching.
An update:
powerpc-linux-gnu-gcc version 5.4.0 (the default package with Ubuntu 16.04)
When using -mregnames, you can use "%r0" or "r0" or "0" format for a register name in assembly source code files.
For disassembling, powerpc-linux-gnu-objdump defaults to the "r0" format (which I agree is easier to read).
In the example from that webpage, it looks like it is showing the listing output from the compiler, instead of using objdump. I do not know of a way to control the listing output format.
I've tried declaring variables in .text segment using e.g. file_handle: dd 0.
However, trying to store something in this variable like mov [file_handle], eax results in a write error.
I know, I could declare writeable variables in the .data segment, but to make the code more compact I'd like to try it as above.
Is the only possibility to use the stack for storing these value (e.g. the file handle), or could I somehow write to my variable above?
Executable code segments are not writable by default. This is a basic security precaution. No, it's not a good idea. But if you insist, as this is a toy project anyway, go ahead.
You can make yours writable by letting the linker know to mark it so, e.g. give the following argument to the MS linker:
link /SECTION:.text,EWR ....
You can actually arrange for the text segment of your Windows process to be mapped read+write+execute, see #Kuba's answer. This might also be possible on Linux with ELF binaries; I think ELF has similar flags for segments.
I think you could also call a Windows function (VirtualProtect) to change the mapping of your text segment to read+write+execute from inside your process.
Overall this sounds like a terrible idea, and you should definitely keep temporaries on the stack like a C compiler would, if you want to avoid having a data page.
Static storage for things you only use in part of the program is wasteful.
No it's not possible to have writable "variable" in .text section of an assembly program.
When writing file_handle: dd 0 in the .text section and then assemblying, your label file_handle refers to an address located in the text section of your binary. However the text section is read-only.
If the text section wasn't only read-only accessible, a program could modify itself while executing.
For my LC3 assignment, I need to enter x3100 for the starting address of the file, how would I do that? Like what opcode would I need, I am not really sure among the ones we've studied so far.
You would use
.ORIG x3100
At the beginning of your code. It's called a pseudo-op or assembler directive, because only the assembler uses it.