Binary values in hexidecimal notation written directly to video memory

Question

In X86 Legacy boot loader prologue the first seven instructions preserve the state of the 8 general purpose and 6 segment registers. This is where they are displayed, right justified without leading zeros. Most often you'll see this kind of thing formulated as an ASCII string, then displayed with printf or maybe if INT 80 or SYSCALL under Linux.

The addresses are absolute and for those that may be interested in executing the code, you can PM me and I'll send the current version that can be assembled with NASM. Then breakpoints can be set and in my case I use BOCH's.

Here is the result; enter image description here SI is already pointing to the beginning of this string

'AXCXDXBXSPBPSIDIESDSCSSSFSGS'

 700 83C61B add si, 27 Point to last character
 703 FD std Do things in reverse
 704 6800B8 push VIDEO_SEG
 707 07 pop es
 708 BF9C09 mov di, 0x99c Row 15 Column 30 (zero indexed)
 70B B90E00 mov cx, 14 Values to be displayed
 70E B407 mov ah, 0x7 in low intensity white

At this point SP is pointing to the values preserved in proglogue.

enter image description here

that is why this instruction without the corresponding push works. After all, is said and done, SS:SP will be pointing to the far pointer back into the BIOS.

 710 5A pop dx Retrieve registers value

Retain pointer to end of hex value (DI) and in case we change attribute which is in AH, preserve it too.

 711 57 push di
 712 50 push ax AH = White
 713 09D2 or dx, dx Is value zero
 715 7502 jnz 719
 717 FEC4 inc ah AH = Grey
 719 E8BFFF call 6db

Now registers mnemonic can be displayed in bright yellow

 71C B40E mov ah, 14 Attribute bright yellow
 71E AC lodsb
 71F AB stosw
 720 AC lodsb
 721 AB stosw
 722 58 pop ax AH = White again
 723 5F pop di
 724 81EFA000 sub di, 160 Bump pointer up one line
 728 FEC9 dec cl 
 72A 740A jz 736 Bail if done ZF=1

As soon as CL = 8, that means all the segment registers have been displayed and we can move over to the next column

 72C 80F908 cmp cl, 8
 72F 75DF jnz 710
 731 BF200A mov di,0xa20 DI = 5DC 
 734 EBDA jmp 710

One thing I had considered here is adding 444H to DI instead, which in effect does exactly the same and then the left column would stay relative if the word at 709H was changed.

Finally, restore DF

 736 FC cld

---

This is where DX will be right justified without leading spaces

 6DB 51 push cx
 6DC B105 mov cl, 0x5
 Converts low nibble in AL to corresponding 0-1 or A-F
 6DE 88D0 mov al, dl
 6E0 240F and al, 0xf
 6E2 3C0A cmp al, 0xa
 6E4 7202 jc 6e8
 6E6 0407 add al, 0x7
 6E8 0430 add al, 0x30
 6EA AB stosw
 6EB FEC9 dec cl
 Asserting DX here assures only one zero will be displayed for zero
 values in DX
 6ED C1EA04 shr dx, 4
 6F0 75EC jnz 6de
 6F2 D1E1 shl cx, 1
 6F4 29CF sub di, cx
 6F6 B82D0F mov ax, 0xf2d
 6F9 66AB stosd Write bright white hyphen
 6FB 59 pop cx
 6FC C3 ret

• David Wohlferd: Retrive, hypen, charcter - If these comments are actually in your code, you might want to fix the spelling, otherwise future maintainers will laugh at you...
• Shift_Left: Actually, sources have no comments at all. I'm experimenting with a different sort of regime through pages done with LibreOffice Impress. That being said, there's still no excuse for errors as there is spellcheck wich I usually use.
• David Wohlferd: Wich - I see what you did there... I'm not sure what you are doing with LibreOffice, but code (especially assembler, but any code) without comments sounds like a support nightmare.
• Shift_Left: @DavidWohlferd Wich reminds me to get a new keyboard, this one can't spell very well. Once I get into the kernel part of my code, I'll probably change things up quite a bit, as it's very likely GRUB will replace this, then maintainability will be much more important. The way binaries are written to fixed, flexible and removable media at the point is monumentally unorthodox.

Answer 1

This is a very nice routine that totally succeeds in using a direction flag set to 1. I've seen others fail at this.

Some observations:

731 BF200A mov di,0xa20 DI = 5DC 

It's not clear what you mean with "DI = 5DC"

One thing I had considered here is adding 444H to DI instead, which in effect does exactly the same and then the left column would stay relative if the word at 709H was changed.

I would definitely choose adding 444h to DI in such a specially crafted routine. Relative addressing is so much more flexible to work with.

6F6 B82D0F mov ax, 0xf2d
6F9 66AB stosd Write bright white hyphen

This is the only place where your code conceivably could fail. You store the dword from EAX without making sure the high word has any meaningful content. If the high word is empty you're in the clear, if not the screen will get disrupted.
Since modern BIOSes do use 32-bit registers, you can't be sure...

The shortest way to 'correct' this is:

mov ax, 0x0F2D
stosw
dec di
dec di

• Shift_Left: Yes, the comment at 731H could have been clearer, but at that point, the routine had just finished displaying contents of ES register and the value in DX = 5DC before mov instruction. So 5DC + 444H = A20H.
• Sep Roland: Right. It's one of those things then that one can only understand through recalculating.
• Shift_Left: At 6F9 EAX = 00000F2D. It should be 07200F2D, but zero to video works just as well and the premise to the instruction was to decrement DI byte 4, ...but... not a very good habit to get into cause this sort of thing is very innocuous on the surface, and monumentally hard to trace when there is a problem. Case in point, I just tried to trace back and couldn't find the place where high order bits of EAX were zeroed. Not a very good commentary, just having finished the app a couple of days ago.