在大会的几行说明 [英] Explanation of a few lines in Assembly

问题描述

bits 16

org 0x7c00

start: jmp loader

;******; ; OEM Parameter block ;********;

TIMES 0Bh-$+start DB 0; THIS LINE

bpbBytesPerSector: DW 512 

bpbSectorsPerCluster: DB 1

bpbReservedSectors: DW 1

bpbNumberOfFATs: DB 2 

bpbRootEntries: DW 224 

bpbTotalSectors: DW 2880 

bpbMedia: DB 0xF0 

bpbSectorsPerFAT: DW 9 

bpbSectorsPerTrack: DW 18

bpbHeadsPerCylinder: DW 2

bpbHiddenSectors: DD 0 

bpbTotalSectorsBig: DD 0 

bsDriveNumber: DB 0 

bsUnused: DB 0 

bsExtBootSignature: DB 0x29 

bsSerialNumber: DD 0xa0a1a2a3 

bsVolumeLabel: DB "MOS FLOPPY "

bsFileSystem: DB "FAT12   "

;******** ; Bootloader Entry Point ;**********;

loader:

cli 

hlt 

times 510 - ($-$$) db 0 

dw 0xAA55 

现在的问题是，我不明白是什么时代0BH - $ +起始计算为在这种情况下。
例如：$ - $$ =大小的程序。
它也将是很有益的，如果有人可以给我LODSB语法讲解。也就是为什么有一个：其中每个BPB和BS陈述后标志？未在：标志意味着一个新的部分的开始，就像在装载部分的情况下，或者在这个方案开始部分。
请详细解释下如何评估答案。

Now the problem is I don't quite understand what TIMES 0bh-$+start evaluates to in this case. For Example $-$$ = size of the program. Also it will be really heĺpful if someone can explain to me the LODSB Syntax. Also why is there a : sign after each of those bpb and bs statements? Doesn't the : sign mean the beginning of a new section, just as in case of loader section or start section in this program. Please explain in detail how the following evaluates to the answer.

感谢。

推荐答案

啊！ 此行只是保留空间OEMNAME。把你的OS的名称出现，如果你喜欢 - 填补到11个字节。 （$$是一样的开始 - 一节的开头 - 0x7C00 - 在这种情况下）。

Ah! "THIS LINE" just reserves space for "OEMNAME". Put the name of your OS there, if you like - padded to 11 bytes. ($$ is the same as "start" - beginning of section - 0x7C00 - in this case).

没有太多的语法来 LODSB ，它只是一个指令......
http://home.myfairpoint.net/fbkotler/nasmdocc.html#section -A.4.141
（为什么我迷惑你使用lodbs作为typoed指令的例子吗？对不起。）

Not much "syntax" to lodsb, it's just an instruction... http://home.myfairpoint.net/fbkotler/nasmdocc.html#section-A.4.141 (did I confuse you using "lodbs" as an example of a typoed instruction? Sorry.)

在：做绝对没问题。在单独线路上的标签的情况下，它会通知NASM它旨在是一个标签，并且不是typoed指令。看手册orphan_label的。 。NASM会（可选 - 默认开启）发出警告，如果没有：，但做正确的事反正

The ":" does absolutely nothing. In the case of a label alone on a line, it informs Nasm that it is intended to be a label, and isn't a typoed instruction. Look in the Manual for "orphan_label". Nasm will (optionally - default ON) warn if there's no ":", but does the right thing anyway.

您真的应该有一个 NOP 在 JMP启动，因为NASM将发出一个短 JMP （旧版本的NASM默认为近 JMP ）。由于THIS LINE的全部目的是为了把剩下的BPB变量在正确的地方，还不如去做！

You really ought to have a nop after jmp start, since Nasm will emit a "short" jmp (older versions of Nasm defaulted to a near jmp). Since the entire purpose of "THIS LINE" is to put the remaining BPB variables in the right place, might as well do it!

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