(hardware) reserved memory & GDT question

[makuc9]

Are you familiar with concept of LITTLE ENDIAN architecture (of which all Intel processors in use are)?

Yes, I am. But doesn't that mean that only quad words, double words, words are written the other way? (at least that's how they were stored in binary files that I've been compiling when working on first stage bootloader).

Anyway...Fixing create memory map function & print memory map function.

[dozniak]

Yes, I am. But doesn't that mean that only quad words, double words, words are written the other way?

Yes, but you only print qwords and dwords in regards to e820 function, so this question seems moot to me.

[makuc9]

Hmmm... Is this better?

Altough I still don't know what the heck is wrong with the first line (second qword)...

I changed each memory map entry size to outputed cl and printing function now prints only qwords and the last dword reversed. Should I reverse the whole entries?

[dozniak]

Hmmm... Is this better?

It looks completely off.
Can you show byte-by-byte hex dump of what e820 returns? (Assuming you are able to write byte printing function)

[Opcode]

Try this

Code: Select all

;ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
;³ FUNCTION     ³ BuildE820Map                                                 ³
;ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
;³ DESCRIPTION  ³ Build an E820 map at ES:DI                                   ³
;ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
;³ ENTRY        ³ ES:DI - Starting location to store the memory map            ³
;³              ³                                                              ³
;³              ³ NOTE: Assumes ES:DI doesn't wrap around                      ³
;ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
;³ EXIT         ³ CF - Set on error                                            ³
;ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
;³ REMARKS      ³ Output For @ ES:DI                                           ³
;³              ³                                                              ³
;³              ³       Count (4 Bytes)      OFFSET      0                     ³
;³              ³       Entry 1 (24 Bytes)   OFFSET     28                     ³
;³              ³       .                                                      ³
;³              ³       .                                                      ³
;³              ³       .                                                      ³
;³              ³       Entry n (24 Bytes)   OFFSET (n * 24) + 4               ³
;³              ³                                                              ³
;ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
BuildE820Map:
	pushad
	mov	DWORD [ES:DI], 0x00000000
	mov	bp, di
	add	di, 4
	xor	ebx, ebx

.nextEntry:
	mov	DWORD [ES:DI + 20], 0x00000001
	mov	eax, 0xE820
	mov	ecx, 24
	mov	edx, 0x0534D4150 ;'SMAP'
	int	0x15
	jc	.exitLoop
	cmp	eax, 0x0534D4150 ;'SMAP'
	jne	.exitLoop
	cmp	ecx, 20
	jl	.exitLoop
	cmp	ecx, 24
	jg	.exitLoop

	; We have a valid entry
	inc	DWORD [ES:BP]
	add	di, 24
	cmp	ebx, 0
	jne	.nextEntry

.exitLoop:
	cmp	DWORD [ES:BP], 0x00000000
	popad
	je	.errorExit
	clc
	ret

.errorExit:
	stc
	ret

[Mikemk]

(Assuming you are able to write byte printing function)

Try this

Code: Select all

...

You're assuming quite a bit. I don't think he knows what it is.

@Opcode, your function trashes the bottom (top) of the stack.

[Opcode]

@Opcode, your function trashes the bottom (top) of the stack.

Where?

[Mikemk]

BP is the base pointer of the stack. changing it effectively moves the stack.

[Brendan]

Hi,

BP is the base pointer of the stack. changing it effectively moves the stack.

For high level languages (e.g. C), BP/EBP/RBP is used as the stack frame pointer (if the compiler's optimiser hasn't been told to omit the frame pointers).

For assembly you're not restricted to HLL conventions - BP/EBP/RBP is just another register and you can do whatever you like with it.


Cheers,

Brendan

[Opcode]

Hehe, you had wondering for a few minutes. I thought you got .errorExit and .exitLoop mixed up.

[dozniak]

Code: Select all

	cmp	ecx, 20
	jl	.exitLoop
	cmp	ecx, 24
	jg	.exitLoop

	; We have a valid entry
	inc	DWORD [ES:BP]
	add	di, 24

Shouldn't you be adding ecx to edi here? Not just 24. Because if ecx was 20 in comparison above, you're just screwing up the map?

Edit: oh, wait, I think I see. So you're putting everything into a fixed-size map, sorry for the confusion.

[makuc9]

Can you show byte-by-byte hex dump of what e820 returns? (Assuming you are able to write byte printing function)

You're assuming quite a bit. I don't think he knows what it is.

Thank you m12, you were really helpful here. I know what a hex dump is and I already posted it once (which I only seperated with spaces where I first thought qwords were) altough it was 24 bytes in length! (But in my function I was increasing destination source register by 24, so entries shouldn't be that much off. I mean first 20 bytes are the same, right? E820h function does not really "care" about ds, it only "cares" about bx value - well, not only about bx, but you should understand what I meant). Than you all were telling me to use little endian encoding while no-one mentioned how to reverse entries: as a whole or as qwords. I also edited MM generation function to use returned CL as a size for each entry and not 24 bytes as previously specified by me. There is not really neccessary for you to insult me, you know. Sure, I just turned 18 (3 weeks ago). Sure I am not learning programming on any programming school (I am at general high school, or secondary school, don't know how is it more proper to say, one of the best in my country) which unfortunately does not cover this topic, only some Microsoft Office Word, PP, Excel and Access crap (and some very basic HTML structures - actually only how to edit existing templates downloaded from the internet). So yea, I've learnt PHP, HTML (& CSS2.1) and C++ to a quite high degree myself and lately (in the passed 1-2 months) I have also being learning Assembly. I did successfully completed the first stage of bootloader for FAT32 (USB) drive (and I do not mean only copy&pasting it - I did made some mistakes in it, you can find my questions about it here, but neverthless, it is completed now), which can load a file from FAT 32 Byte Directory Entry Structure (I guess only up to 480KB in size - Yes, I load it in 0x00007E00 address which is only 480.5KB in size) and pass control to it. I have also programmed some other, different kinds of testing stuff to get more accustumed to assembly (also number printing function for numbers higher than 9 and as you can see also in hex number - hex printing even displays numbers in even format - adds preleading zero). I am only not yet that familiar with x86 & x64 architectures, so just correct me if I am wrong, no need to insult me (I am also not a native english speaker, so I DO confuse some explaination and misunderstand others).

Anyway, will return here tomorrow (I have to continue preparing myself for examination which I have tomorrow, so I only came online to check emails and check responses here, because I got alerted by mail notification).

Regards.

[Mikemk]

There is not really neccessary for you to insult me, you know.

I never insulted you.

Sure, I just turned 18 (3 weeks ago).

Firstly, you're older than me. This, however, is irrelevant.

Sure I am not learning programming on any programming school

That's the best way to learn. (No joke here).

(I am at general high school, one of the best in my country) which unfortunately does not cover this topic, only some Microsoft Office Word, PP, Excel and Access crap (and some very basic HTML structures - actually only how to edit existing templates downloaded from the internet).

More than my high school. We're stuck with Open Office, and no programming. I taught myself everything I know.

[iansjack]

BP is the base pointer of the stack. changing it effectively moves the stack.

Hmm.

I taught myself everything I know.

It's very honest of you to take the blame.

[Mikemk]

lol @ iansjack. That actually came out of Stack. Rereading through that, I guess I misunderstood/misread.