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......
（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!
这篇关于在大会的几行说明的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！