I need a pointer to location which is always the same. So, how can I create a pointer to.. lets say memory address 0x20 and store it in some way to be able to access it later.
Note:
I do NOT want to store the result, but the actual pointer to the memory address (since I want to point to the beginning of an array).
Thanks in advance.
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I think I have fixed it now. I use bios interrupt 0x15 to get a memory map. Every interrupt returns 1 entry and you provide a pointer in es:di to a place where the bios can store it. I let the bios build it up from 050h:0h. I needed a pointer to 0x50:0x0 (0x500 linear) to use the map later. I still have to test, but I did the following:
mov ax, 0x50
mov es, ax
xor di, di
shl ax, 4
add ax, di
mov [mmr], ax
And mmr is declared this way:
mmr:
dw 0 ; pointer to the first entry
db 0 ;entry count
db 24 ; entry size
A pointer is just a memory address and a memory address is just a number. Assembly is not a typed language so there is no difference.
Also assembly doesn't really have variables. It has registers and memory locations, both of which can be used for storing values, including addresses/pointers.
So basically there are many variants of the x86 MOV instruction that can store a pointer such as 0x20 in an address or a register. You certainly want to think about whether you're doing 32-bit or 64-bit x86 assembly though (or 16-bit or even 8-bit for that matter).
x86:
suppose you have an array called list
mov bx, offset list
now, in the bx register you will have a pointer to the first memory location of list
to refer to the data in the memory location you would use [bx]
here's an brief example using intel syntax:
;declare list in .data
list dw 0123h
;move 01h from memory to ax register (16-bit)
mov bx, offset list
mov al, [bx] ; al = 23h
If you want to use the pointer later you can do this:
push bx then pop bx when you want to use it
or
mov point, bx ; declared in mmr
Related
I am trying to assemble the code below using yasm. I have put 'here' comments where yasm reports the error "error: invalid size for operand 2". Why is this error happening ?
segment .data
a db 25
b dw 0xffff
c dd 3456
d dq -14
segment .bss
res resq 1
segment .text
global _start
_start:
movsx rax, [a] ; here
movsx rbx, [b] ; here
movsxd rcx, [c] ; here
mov rdx, [d]
add rcx, rdx
add rbx, rcx
add rax, rbx
mov [res], rax
ret
For most instructions, the width of the register operand implies the width of the memory operand, because both operands have to be the same size. e.g. mov rdx, [d] implies mov rdx, qword [d] because you used a 64-bit register.
But the same movsx / movzx mnemonics are used for the byte-source and word-source opcodes, so it's ambiguous unless the source is a register (like movzx eax, cl). Another example is crc32 r32, r/m8 vs. r/m16 vs. r/m32. (Unlike movsx/zx, its source size can be as wide as the operand-size.)
movsx / movzx with a memory source always need the width of the memory operand specified explicitly.
The movsxd mnemonic is supposed to imply a 32-bit source size. movsxd rcx, [c] assembles with NASM, but apparently not with YASM. YASM requires you to write dword, even though it doesn't accept byte, word, or qword there, and it doesn't accept movsx rcx, dword [c] either (i.e. it requires the movsxd mnemonic for 32-bit source operands).
In NASM, movsx rcx, dword [c] assembles to movsxd, but movsxd rcx, word [c] is still rejected. i.e. in NASM, plain movsx is fully flexible, but movsxd is still rigid. I'd still recommend using dword to make the width of the load explicit, for the benefit of humans.
movsx rax, byte [a]
movsx rbx, word [b]
movsxd rcx, dword [c]
Note that the "operand size" of the instruction (as determined by the operand-size prefix to make it 16-bit, or REX.W=1 to make it 64-bit) is the destination width for movsx / movzx. Different source sizes use different opcodes.
In case it's not obvious, there's no movzxd because 32-bit mov already zero-extends to 64-bit implicitly. movsxd eax, ecx is encodeable, but not recommended (use mov instead).
In AT&T syntax, you need to explicitly specify both the source and destination width in the mnemonic, like movsbq (%rsi), %rax. GAS won't let you write movsb (%rsi), %eax to infer a destination width (operand-size) because movsb/movsw/etc are the mnemonics for string-move instructions with implicit (%rsi), (%rdi) operands.
Fun fact: GAS and clang do allow it for things like movzb (%rsi), %eax as movzbl, but GAS only has extra logic to allow disambiguation (not just inferring size) based on operands when it's necessary, like movsd (%rsi), %xmm0 vs. movsd. (Clang12.0.1 actually does accept movsb (%rcx), %eax as movsbl, but GAS 2.36.1 doesn't, so for portability it's best to be explicit with sign-extension, and not a bad idea for zero-extension too.)
Other stuff about your source code:
NASM/YASM allow you to use the segment keyword instead of section, but really you're giving ELF section names, not executable segment names. Also, you can put read-only data in section .rodata (which is linked as part of the text segment). What's the difference of section and segment in ELF file format.
You can't ret from _start. It's not a function, it's your ELF entry point. The first thing on the stack is argc, not a valid return address. Use this to exit cleanly:
xor edi,edi
mov eax, 231
syscall ; sys_exit_group(0)
See the x86 tag wiki for links to more useful guides (and debugging tips at the bottom).
I have two two-dimensional arrays with dynamic sizes (guess that's the proper wording). I copy the content of first one into the other using:
dest:=copy(src,0,4*x*y);
// src,dest:array of array of longint; x,y:longint;
// setlength(both arrays,x,y); //x and y are max 15 bit positive!
It works. However I'm unable to reproduce this in asm. I tried the following variations to no avail... Could someone enlighten me...
MOV ESI,src; MOV EDI,dest; MOV EBX,y; MOV EAX,x; MUL EBX;
PUSH DS; POP ES; MOV ECX,EAX; CLD; REP MOVSD;
Also tried with LEA (didn't expect that to work since it should fetch the pointer address not the array address), no workie, and tried with:
p1:=#src[0,0]; p2:=#dest[0,0]; //being no-type pointers
MOV ESI,p1; MOV EDI,p2... (the same asm)
Hints pls? Btw it's delphi 6. The error is, of course, access violation.
This is really a two-fold three-fold question.
What's the structure of a dynamic array.
Which instructions in asm will copy the array.
I'm throwing random assembler at the CPU, why doesn't it work?
Structure of a dynamic array
See: http://docwiki.embarcadero.com/RADStudio/Seattle/en/Internal_Data_Formats
To quote:
Dynamic Array Types
On the 32-bit platform, a dynamic-array variable occupies 4 bytes of memory (and 8 bytes on 64-bit) that contain a pointer to the dynamically allocated array. When the variable is empty (uninitialized) or holds a zero-length array, the pointer is nil and no dynamic memory is associated with the variable. For a nonempty array, the variable points to a dynamically allocated block of memory that contains the array in addition to a 32-bit (64-bit on Win64) length indicator and a 32-bit reference count. The table below shows the layout of a dynamic-array memory block.
Dynamic array memory layout (32-bit and 64-bit)
Offset 32-bit -8 -4 0
Offset 64-bit -12 -8 0
contents refcount count start of data
So the dynamic array variable is a pointer to the middle of the above structure.
How do I access this in asm
Let's assume the array holds records of type TMyRec
you'll need to run this code for every inner array in the outer array to do the deep copy. I leave this as an exercise for the reader. (you can do the other part in pascal).
type
TDynArr: array of TMyRec;
procedure SlowButBasicMove(const Source: TDynArr; var dest);
asm
//insert register pushes, see below.
mov esi,Source //esi = pointer to source data
mov edi,Dest //edi = pointer to dest
sub esi,8
mov ebx,[esi] //ebx = refcount (just in case)
mov ecx,[esi+4] //ecx = element count
mov edx,SizeOf(TMyRec) //anywhere from 1 to zillions
mul ecx,edx //==ecx=number of bytes in array.
//// now we can start moving
xor ebx,ebx //ebx =0
add eax,8 //eax = #data
#loop:
mov eax,[esi+ebx] //Get data from source
mov [edi+ebx],esi //copy it to dest
add ebx,4 //4 bytes at a time
cmp ebx,ecx //is ebx> number of bytes?
jle loop
//Done copying.
//insert register pops, see below
end;
That's the copy done, however in order for the system not to crash, you need to save and restore the non volatile registers (all but EAX, ECX, EDX), see: http://docwiki.embarcadero.com/RADStudio/Seattle/en/Program_Control
push ebx
push esi
push edi
--- insert code shown above
//restore non-volatile registers
pop edi
pop esi
pop ebx //note the restoring must happen in the reverse order of the push.
See the Jeff Dunteman's book assembly step by step if you're completely new to asm.
You will get access violations if:
you try to read from a wrong address.
you try to write to a wrong adress.
you read past the end of the array.
you write to memory you haven't claimed before using GetMem or whatever means.
if you write past the end of your buffer.
if you do not restore all non-volatile registers
Remember you're directly dealing with the CPU. Delphi will not assist you in any way.
Really fast code will use some form of SSE to move 16bytes per instruction in an unrolled loop, see the above mentioned fastcode for examples of optimized assembler.
Random assembler
In assembler you need to know exactly what you're what to do, how and what the CPU does.
Set a breakpoint and run your code. Press ctrl + alt + C and behold the CPU-debug window.
This will allow you to see the code Delphi generates.
You can single step through the code to see what the CPU does.
see: http://www.plantation-productions.com/Webster/index.html
For more reading.
Dynamic Arrays differ from Static Arrays, especially when it comes to multi-dimensionality.
Refer to this reference for internal formats.
The point is that an Array Of Array Of LongInt of dimensions X and Y (in this order!) is a pointer to an array of X pointers that point to an array of Y LongInts.
Since it seems, from your comments, that you have already allocated the space for all elements in Dest, I assume you want to do a Deep Copy.
Here a sample program, where the assembly as been made as simple as possible for the sake of clarity.
Program Test;
Var Src, Dest: Array Of Array Of LongInt;
X, Y, I, J: Integer;
Begin
X := 4;
Y := 2;
setLength(Src, X, Y);
setLength(Dest, X, Y);
For I := 0 To X-1 Do
For J := 0 To Y-1 Do
Src[I,J] := I*Y + J;
{$ASMMODE intel}
Asm
cld ;Be sure movsd increments the registers
mov esi, DWORD PTR [Src] ;Src pointer
mov edi, DWORD PTR [Dest] ;Dest pointer
mov ecx, DWORD PTR [X] ;Repeat for X times
;The number of elements in Src
#_copy:
push esi ;Save these for later
push edi
push ecx
mov ecx, DWORD PTR [Y] ;Repeat for Y times
;The number of element in a Src[i] array
mov edi, DWORD PTR [edi] ;Get the pointer to the Dest[i] array
mov esi, DWORD PTR [esi] ;Get the pointer to the Src[i] array
rep movsd ;Copy sub array
pop ecx ;Restore
pop edi
pop esi
add esi, 04h ;Go from Src[i] to Src[i+1]
add edi, 04h ;Go from Dest[i] to Dest[i+1]
loop #_copy
End;
For I := 0 To X-1 Do
Begin
WriteLn();
For J := 0 To Y-1 Do
Begin
Write(' ');
Write(Dest[I,J]);
End;
End;
End.
Note 1 This source code is intended to be compile with freepascal.
Donation of Spare Time(TM) for downloading and installing Delphi are welcome!
Note 2 This source code is for illustration purpose only, it is pretty obvious, it has already been stated above, but somehow not everybody got it.
If the OP wanted a fast way to copy the array, they should have stated so.
Note 3 I don't save the clobbered registers, this is bad practice, my bad; I forgot, as there are no subroutines, no optimizations and no reason for the compiler to pass data in the registers between the two fors.
This is left as an exercise to the reader.
Let's say for example I have four specific memory addresses that each hold a 32-bit integer. How would you use assembly language to take the address and assign it register %eax?
Would it be movl 0x12AED567, %eax?
Yes, it is that simple. If you already have the addresses, just assign them to eax, I corrected your code a little :
mov 12AED567h, eax
But, if you want to get the addresses dynamically, you have to use lea instruction, next little program shows how :
.stack 100h
.data
my_number dd A01Ch
.code
;INITIALIZE DATA SEGMENT.
mov ax,#data
mov ds,ax
;GET THE MEMORY ADDRESS OF MY_NUMBER, NOT THE NUMBER ITSELF.
lea eax, my_number
;FINISH THE PROGRAM PROPERLY.
mov ax,4c00h
int 21h
Is this what you were looking for?
By the way, this is 8086 assembler with Intel's syntax.
How do you perform operations like change the value at an absolute word address?
Say you have some value at 5DAh and you want to count the number of zeros on that address, or move a value from one absolute address to another. How can one do that?
Short Answer: You Can't
You might have a trick question in front of you (no clue, just my guess).
The physical architecture of the 8086 chip did not have that instruction.
As for your two specific questions...
"...you want to count the number of zeros on that address..."
That's somewhat ambiguous, in fact so vague that I can't understand it.
"...move a value from one absolute address to another..."
Good question. We'll do this in 32 bit, no, 16 and then 32 bit.
16 bit example
Push Si ;source index register
Push Di ;destination index register
Push Ax ;We'll use this for the transfer
Lea Si, Where_The_Number_Is_Now ;You'll define this, somehow
Lea Di, Where_We_Want_It_To_Go ;You'll define this also, same thing
Mov Ax, Ds:[Si] ;The "Ds:" may or may not be needed, be safe
Mov Ds:[Di], Ax ;Probably do need "Ds:" for this instruction
Pop Ax ;Do pay attention to the reverse order
Pop Di ;...of popping the registers in exact
Pop Si ;...opposite of how they were pushed
; And you are done
32 bit example
Push Esi ;source index register
Push Edi ;destination index register
Push Eax ;We'll use this for the transfer
Lea Esi, Where_The_Number_Is_Now ;You'll define this, somehow
Lea Edi, Where_We_Want_It_To_Go ;You'll define this also, same thing
Mov Eax, Ds:[Esi] ;The "Ds:" may or may not be needed, be safe
Mov Ds:[Edi], Eax ;Probably do need "Ds:" for this instruction
Pop Eax ;Do pay attention to the reverse order
Pop Edi ;...of popping the registers in exact
Pop Esi ;...opposite of how they were pushed
; And you are done
To change a value at an absolute word address:
mov byte ptr [5dah], 0
...or...
mov word ptr [5dah], 0
To move a value from one absolute word address to another:
mov al, byte ptr [5dah]
mov byte ptr [1234h], al
...or...
mov ax, word ptr [5dah]
mov word ptr [1234h], ax
As for the other question, the one that asked how to count the number of zeros on that address, you were a little to vague.
I want to write a program for displaying ascii symbols by writing directly to the video memory.
.model tiny
.stack
.data
.code
main:
mov ax, b800h
mov ds, ax
mov aL, 'x'
mov es, bx
mov es:[20], aL
end main
I'm getting this error when i try to compile with TASM
"Undefined symbol: B800H"
Put a 0 in front of it so TASM can tell it's a numeric value rather than a symbol:
mov ax, 0b800h
You also appear to be missing anything that will return from your program. Without that, it's hard to say what code will be run after the mov es:[20], al, but I suspect it won't be pretty.
You need something like int 21, function 4c. I've always found Ralf Brown's interrupt list to be invaluable for this sort of stuff.
You may also want to consider the possibility of moving to a tool that's been in active maintenance sometime in the last decade :-)
TASM 5 was released in the 90s and patched up to about 2002, and doesn't support a lot of the new stuff that's been done since then (MMX, SSE and so on). My personal favorite is NASM but there are a number of other options.