experchange > asm.x86

bilsch01 (12-26-19, 11:44 AM)
The code below writes 5 words to memory at 0x37000. I verified the words
are there using code not shown (for simplicity), but it's easy to see
that the words should be there. After that the 2 lines:

mov si,3
mov ax,word[fs:si]

should put dd66 in ax register but instead it puts a byte from each of 2
other words in ax, specifically 77bb. The routine at the end prints the
result 77BB when it should print DD66.

I have a situation where I really need something like this to work. How
can I do it? TIA. Bill S.

bits 16
org 0x0000

SECTION .data

fat5 dw 0xaa99,0xbb88,0xcc77,0xdd66,0xee55,0xff44

SECTION .text

push 0
pop ss
mov sp,0x7aff
push 0x1000
pop ds
push 0xb800
pop es

; write 5 words to mem at 0x37000

push 0x3700
pop fs
mov di,0
mov si,fat5
mov cx,5
fatlu: mov ax,word[ds:si]
mov word[fs:di],ax
add si,2
add di,2
loop fatlu

; here's the test

mov si,3
mov ax,word[fs:si]

push ax
jmp vuhex ;prints 77BB
;should be DD66

; prints 4 hex digits from stack

vuhex: pop ax ;4 hex digits
mov di,3832 ;l.r. corner
mov cx,4 ;;# of hex digits
vh01: rol ax,4 ;hi digit on right
push ax ;push rol'd ax
and al,0x0f ;;masks right digit
add al,'0' ;0 thru 9
cmp al,'9'
jbe vh02
add al,0x7 ;A thru F
vh02: mov byte[es:di],al
inc di
mov byte[es:di],0x0f ;blk/wht
inc di
pop ax ;pop rol'd ax
loop vh01

jmp $
rick.c.hodgin (12-26-19, 12:30 PM)
On Thursday, December 26, 2019 at 4:53:21 AM UTC-5, bilsch01 wrote:
[..]
> add si,2
> add di,2
> loop fatlu


You can use ds:[si] and es:[di] and some CPU features if you
want:

push 0x1000
pop ds
push 0x3700
pop es

mov si,fat5
xor di,di
mov cx,5

; May need to set the direction flag here
==> rep movsw

The one line removes several lines of loop code, provided you
are able to use ds: and es: instead of ds: and fs:.

[..]
> pop ax ;pop rol'd ax
> loop vh01
> jmp $


Can you run this code in DOS and verify it's working there?
You can use a debugger to assist you, and it won't matter if
you destroy your runtime environment in DOS because you can
just reboot once you get what information you need from the
debugger. I used to do things like this in the CodeView 3.x
debugger as it was faster than the 4.x series.

And then for my OS project, I wrote a tiny boot-time debugger.
It was essentially two 64 KB modules, one handling the debugger
logic, the other handling the disassembler.

I added some extra code at the start of my 512 KB boot sector
to read in the debugger and disassembler, each which existed
at a fixed location on disk, and read into a fixed locations
in memory. After loading, I'd call the setup function and it
would hook into the appropriate interrupt vectors, and then
return and then I continue to boot up like normal.

It added about 70 bytes of code to my boot sector, but then I
had a full debugger available at boot time. I could single-
step through my code, examine memory, alter memory, etc.

It only worked in 16-bit mode, as after I entered 32-bit mode
I had my own protected mode kernel debugger available, and
could use that.

I recommend finding / creating something like that to help you
out with all of your boot-time assembly needs. The real-mode
debugger I wrote was able to be run and tested in DOS as a .COM
file using a custom loader to simulate the boot-time load. I
used it to debug itself at various times. :-)

-----
I have the source code for my debugger if you're interested.
I called it xDebug. It was written entirely in MASM 6.11d. I
can put it up on my website if you'd like to download it. If
so, let me know.
s_dubrovich (12-26-19, 06:29 PM)
On Thursday, December 26, 2019 at 3:53:21 AM UTC-6, bilsch01 wrote:
[..]
> pop ax ;pop rol'd ax
> loop vh01
> jmp $


SECTION .data

fat5 dw 0xaa99,0xbb88,0xcc77,0xdd66,0xee55,0xff44

;; prints 77BB, should be DD66

;; fat5: db 99h, 0AAh, 88h, [0BBh, 77h], 0CCh, {66h, 0DDh}, 55h, 0EEh, 44h, 0FFh

;; SI = 3 points to [nnnnh], as db 99h is at index (offset) 0

'[]' brackets the actual result
'{}' brackets the desired result

SECTION .text

; mov si, 3
; mov ax, word[fs:si]

;; SI needs to be a word index (offset), 3*2, to point to {nnnnh}
;; SI = 0000h points to byte 99h, plus 6 then points to the low order byte 66h,
;; the high ordered byte is DDh.
;; The correction is:
;;
;; mov si, 3*2

Steve
bilsch01 (12-27-19, 11:08 AM)
On 12/26/19 8:29 AM, s_dubrovich wrote:
[..]
> ;;
> ;; mov si, 3*2
> Steve


Thanks for your explanation, I realized my mistaken thinking as soon as
I saw it.
Kerr-Mudd, John (01-18-20, 07:05 PM)
On Thu, 26 Dec 2019 09:30:31 -0100, <rick.c.hodgin>
wrote:

> On Thursday, December 26, 2019 at 4:53:21 AM UTC-5, bilsch01 wrote:
>> The code below writes 5 words to memory at 0x37000. I verified the words


[]

> I recommend finding / creating something like that to help you
> out with all of your boot-time assembly needs. The real-mode
> debugger I wrote was able to be run and tested in DOS as a .COM
> file using a custom loader to simulate the boot-time load. I
> used it to debug itself at various times. :-)
> -----
> I have the source code for my debugger if you're interested.
> I called it xDebug. It was written entirely in MASM 6.11d. I
> can put it up on my website if you'd like to download it. If
> so, let me know.


I'd like to see your xdebug code.
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