7 ms·

Why do PCs load their boot sector at 7C000h linear?

73 points by monopolemagnet 11y ago

yuhong 11y ago
It is 7C00h linear.
- raverbashing 11y ago
  Yeah, it's not 7C00:0000, it's 007C:0000 (or 0000:7C00) shudders
  - wruza 11y ago
    07C0:0, actually :)
    - raverbashing 11y ago
      True, too bad I can't fix in the original comment "The 16-bit segment selector in the segment register is interpreted as the most significant 16 bits of a linear 20-bit address" However, Wikipedia has a funny quote on that "Once the BIOS has found a bootable device it loads the boot sector to linear address 7C00h (usually segment:offset 0000h:7C00h, but some BIOSes erroneously use 07C0h:0000h" https://en.wikipedia.org/wiki/Booting#BOOT-LOADER https://en.wikipedia.org/wiki/Booting#BOOT-LOADER
TerryADavis 11y ago
http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html
- Peaker 11y ago
  Do you intentionally leave out parens in the #define for MODULE_SIZE? If it was used in a context with higher precedence than multiplication, it'd be a bug.
2sk21 11y ago
I love this kind of software archeology. I think I first came across this term in Vernor Vinge's book "A Deepness in the Sky".
- pavedwalden 11y ago
  I'm not sure which Zones Of Thought book it was (probably Deepness in the Sky), but my favorite software joke in SciFi is when someone deep-diving into the system discovers that at the deepest layers timekeeping is still calculated from the UNIX epoch on the long lost human homeworld.
  - acheron 11y ago
    Reminds me of this: http://abstrusegoose.com/323 http://abstrusegoose.com/323
- madez 11y ago
  I wouldn‘t call it software archeology because it is still very much in use and modern standard.
roghummal 11y ago
TerryADavis might have something meaningful to say here if given the chance. Can YC selectively un-auto-[dead] people?
- eru 11y ago
  Quoting his comment: TerryADavis 2 hours ago [dead] http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html ---- [End of Quote] Thanks, Terry!
  - buffoon 11y ago
    And today I learn about Terry Davis, TempleOS and showdead. Moderation and censorship is a travesty online. Fortunately showdead allows us to bypass it somewhat.
    - eru 11y ago
      There's a distinction between going out and censoring people, and not letting them post anything they want on your domain.
      - buffoon 11y ago
        No there is not.
        eru 11y ago
        I'll book a trip to your house and write on your walls, then.
        buffoon 11y ago
        I have kids. I'm already there.
userbinator 11y ago
I feel old for recognising that address and knowing the answer to the question before I even read the article... You can see the actual bootloader code in the PC's BIOS here: http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/1981-08/pages/IBM-5150-TECHREF%20214.pdf http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/19... The address is mentioned in lines 44 and 45 here: http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/1981-08/pages/IBM-5150-TECHREF%20196.pdf http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/19... (It would be so very awesome if this was the sort of documentation you got if you purchased a new computer, although perhaps on a disc instead of hardcopy.)
fogleman 11y ago
So it was the same guy who came up with Ctrl+Alt+Del.
MatthewWilkes 11y ago
Can someone provide a TL;DR? I'm finding it very hard to understand the article.
- Dylan16807 11y ago
  IBM's BIOS put the boot loader at the end of the minimum amount of memory at the time (32KB). 7C00-7DFF get the boot sector, and 7E00-7FFF are scratch space. Also once upon a time SCP tucked it into 0200-03FF because that ram was otherwise unused.
  - yuhong 11y ago
    the minimum was 16KB I think (they used 4116s).
    - Dylan16807 11y ago
      Well, I'm just summarizing. > "DOS 1.0 required a minimum of 32KB, so we weren't concerned about attempting a boot in 16KB." > (Note: DOS 1.0 required 16KiB minimum ? or 32KiB ? I couldn't find out which correct. But, at least, in 1981's early BIOS development, they supposed that 32KiB is DOS minimum requirements.)
      - yuhong 11y ago
        There are different configs of IBM PC, the cheapest which relied on the ROM BASIC and cassette tape.
        Dylan16807 11y ago
        Right, but we're worried here about the ones booting DOS, while they were designing this.
deleted 11y ago
[deleted]

It is 7C00h linear.

raverbashing 11y ago
Yeah, it's not 7C00:0000, it's 007C:0000 (or 0000:7C00) shudders
- wruza 11y ago
  07C0:0, actually :)
  - raverbashing 11y ago
    True, too bad I can't fix in the original comment "The 16-bit segment selector in the segment register is interpreted as the most significant 16 bits of a linear 20-bit address" However, Wikipedia has a funny quote on that "Once the BIOS has found a bootable device it loads the boot sector to linear address 7C00h (usually segment:offset 0000h:7C00h, but some BIOSes erroneously use 07C0h:0000h" https://en.wikipedia.org/wiki/Booting#BOOT-LOADER https://en.wikipedia.org/wiki/Booting#BOOT-LOADER

http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html

Peaker 11y ago
Do you intentionally leave out parens in the #define for MODULE_SIZE? If it was used in a context with higher precedence than multiplication, it'd be a bug.

2sk21 11y ago

I love this kind of software archeology. I think I first came across this term in Vernor Vinge's book "A Deepness in the Sky".

pavedwalden 11y ago
I'm not sure which Zones Of Thought book it was (probably Deepness in the Sky), but my favorite software joke in SciFi is when someone deep-diving into the system discovers that at the deepest layers timekeeping is still calculated from the UNIX epoch on the long lost human homeworld.
- acheron 11y ago
  Reminds me of this: http://abstrusegoose.com/323 http://abstrusegoose.com/323
madez 11y ago
I wouldn‘t call it software archeology because it is still very much in use and modern standard.

roghummal 11y ago

TerryADavis might have something meaningful to say here if given the chance. Can YC selectively un-auto-[dead] people?

eru 11y ago
Quoting his comment: TerryADavis 2 hours ago [dead] http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootMHD2.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html http://www.templeos.org/Wb/Adam/Opt/Boot/BootHD.html ---- [End of Quote] Thanks, Terry!
- buffoon 11y ago
  And today I learn about Terry Davis, TempleOS and showdead. Moderation and censorship is a travesty online. Fortunately showdead allows us to bypass it somewhat.
  - eru 11y ago
    There's a distinction between going out and censoring people, and not letting them post anything they want on your domain.
    - buffoon 11y ago
      No there is not.
      - eru 11y ago
        I'll book a trip to your house and write on your walls, then.
        buffoon 11y ago
        I have kids. I'm already there.

userbinator 11y ago

I feel old for recognising that address and knowing the answer to the question before I even read the article... You can see the actual bootloader code in the PC's BIOS here: http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/1981-08/pages/IBM-5150-TECHREF%20214.pdf http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/19... The address is mentioned in lines 44 and 45 here: http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/1981-08/pages/IBM-5150-TECHREF%20196.pdf http://static.pcjs.org/pubs/pc/reference/ibm/5150/techref/19... (It would be so very awesome if this was the sort of documentation you got if you purchased a new computer, although perhaps on a disc instead of hardcopy.)

fogleman 11y ago

So it was the same guy who came up with Ctrl+Alt+Del.

MatthewWilkes 11y ago

Can someone provide a TL;DR? I'm finding it very hard to understand the article.

Dylan16807 11y ago
IBM's BIOS put the boot loader at the end of the minimum amount of memory at the time (32KB). 7C00-7DFF get the boot sector, and 7E00-7FFF are scratch space. Also once upon a time SCP tucked it into 0200-03FF because that ram was otherwise unused.
- yuhong 11y ago
  the minimum was 16KB I think (they used 4116s).
  - Dylan16807 11y ago
    Well, I'm just summarizing. > "DOS 1.0 required a minimum of 32KB, so we weren't concerned about attempting a boot in 16KB." > (Note: DOS 1.0 required 16KiB minimum ? or 32KiB ? I couldn't find out which correct. But, at least, in 1981's early BIOS development, they supposed that 32KiB is DOS minimum requirements.)
    - yuhong 11y ago
      There are different configs of IBM PC, the cheapest which relied on the ROM BASIC and cassette tape.
      - Dylan16807 11y ago
        Right, but we're worried here about the ones booting DOS, while they were designing this.

deleted 11y ago

[deleted]