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<html><head><title>NASM Manual</title></head><body><h1 align=center>The Netwide Assembler: NASM</h1><p align=center><a href="nasmdo10.html">Next Chapter</a> |<a href="nasmdoc8.html">Previous Chapter</a> |<a href="nasmdoc0.html">Contents</a> |<a href="nasmdoci.html">Index</a><h2><a name="chapter-9">Chapter 9: Mixing 16 and 32 Bit Code</a></h2><p>This chapter tries to cover some of the issues, largely related tounusual forms of addressing and jump instructions, encountered when writingoperating system code such as protected-mode initialisation routines, whichrequire code that operates in mixed segment sizes, such as code in a 16-bitsegment trying to modify data in a 32-bit one, or jumps betweendifferent-size segments.<h3><a name="section-9.1">9.1 Mixed-Size Jumps</a></h3><p>The most common form of mixed-size instruction is the one used whenwriting a 32-bit OS: having done your setup in 16-bit mode, such as loadingthe kernel, you then have to boot it by switching into protected mode andjumping to the 32-bit kernel start address. In a fully 32-bit OS, thistends to be the <em>only</em> mixed-size instruction you need, sinceeverything before it can be done in pure 16-bit code, and everything afterit can be pure 32-bit.<p>This jump must specify a 48-bit far address, since the target segment isa 32-bit one. However, it must be assembled in a 16-bit segment, so justcoding, for example,<p><pre> jmp 0x1234:0x56789ABC ; wrong!</pre><p>will not work, since the offset part of the address will be truncated to<code><nobr>0x9ABC</nobr></code> and the jump will be an ordinary 16-bitfar one.<p>The Linux kernel setup code gets round the inability of<code><nobr>as86</nobr></code> to generate the required instruction bycoding it manually, using <code><nobr>DB</nobr></code> instructions. NASMcan go one better than that, by actually generating the right instructionitself. Here's how to do it right:<p><pre> jmp dword 0x1234:0x56789ABC ; right</pre><p>The <code><nobr>DWORD</nobr></code> prefix (strictly speaking, it shouldcome <em>after</em> the colon, since it is declaring the <em>offset</em>field to be a doubleword; but NASM will accept either form, since both areunambiguous) forces the offset part to be treated as far, in the assumptionthat you are deliberately writing a jump from a 16-bit segment to a 32-bitone.<p>You can do the reverse operation, jumping from a 32-bit segment to a16-bit one, by means of the <code><nobr>WORD</nobr></code> prefix:<p><pre> jmp word 0x8765:0x4321 ; 32 to 16 bit</pre><p>If the <code><nobr>WORD</nobr></code> prefix is specified in 16-bitmode, or the <code><nobr>DWORD</nobr></code> prefix in 32-bit mode, theywill be ignored, since each is explicitly forcing NASM into a mode it wasin anyway.<h3><a name="section-9.2">9.2 Addressing Between Different-Size Segments</a></h3><p>If your OS is mixed 16 and 32-bit, or if you are writing a DOS extender,you are likely to have to deal with some 16-bit segments and some 32-bitones. At some point, you will probably end up writing code in a 16-bitsegment which has to access data in a 32-bit segment, or vice versa.<p>If the data you are trying to access in a 32-bit segment lies within thefirst 64K of the segment, you may be able to get away with using anordinary 16-bit addressing operation for the purpose; but sooner or later,you will want to do 32-bit addressing from 16-bit mode.<p>The easiest way to do this is to make sure you use a register for theaddress, since any effective address containing a 32-bit register is forcedto be a 32-bit address. So you can do<p><pre> mov eax,offset_into_32_bit_segment_specified_by_fs mov dword [fs:eax],0x11223344</pre><p>This is fine, but slightly cumbersome (since it wastes an instructionand a register) if you already know the precise offset you are aiming at.The x86 architecture does allow 32-bit effective addresses to specifynothing but a 4-byte offset, so why shouldn't NASM be able to generate thebest instruction for the purpose?<p>It can. As in <a href="#section-9.1">section 9.1</a>, you need onlyprefix the address with the <code><nobr>DWORD</nobr></code> keyword, and itwill be forced to be a 32-bit address:<p><pre> mov dword [fs:dword my_offset],0x11223344</pre><p>Also as in <a href="#section-9.1">section 9.1</a>, NASM is not fussyabout whether the <code><nobr>DWORD</nobr></code> prefix comes before orafter the segment override, so arguably a nicer-looking way to code theabove instruction is<p><pre> mov dword [dword fs:my_offset],0x11223344</pre><p>Don't confuse the <code><nobr>DWORD</nobr></code> prefix<em>outside</em> the square brackets, which controls the size of the datastored at the address, with the one <code><nobr>inside</nobr></code> thesquare brackets which controls the length of the address itself. The twocan quite easily be different:<p><pre> mov word [dword 0x12345678],0x9ABC</pre><p>This moves 16 bits of data to an address specified by a 32-bit offset.<p>You can also specify <code><nobr>WORD</nobr></code> or<code><nobr>DWORD</nobr></code> prefixes along with the<code><nobr>FAR</nobr></code> prefix to indirect far jumps or calls. Forexample:<p><pre> call dword far [fs:word 0x4321]</pre><p>This instruction contains an address specified by a 16-bit offset; itloads a 48-bit far pointer from that (16-bit segment and 32-bit offset),and calls that address.<h3><a name="section-9.3">9.3 Other Mixed-Size Instructions</a></h3><p>The other way you might want to access data might be using the stringinstructions (<code><nobr>LODSx</nobr></code>,<code><nobr>STOSx</nobr></code> and so on) or the<code><nobr>XLATB</nobr></code> instruction. These instructions, since theytake no parameters, might seem to have no easy way to make them perform32-bit addressing when assembled in a 16-bit segment.<p>This is the purpose of NASM's <code><nobr>a16</nobr></code> and<code><nobr>a32</nobr></code> prefixes. If you are coding<code><nobr>LODSB</nobr></code> in a 16-bit segment but it is supposed tobe accessing a string in a 32-bit segment, you should load the desiredaddress into <code><nobr>ESI</nobr></code> and then code<p><pre> a32 lodsb</pre><p>The prefix forces the addressing size to 32 bits, meaning that<code><nobr>LODSB</nobr></code> loads from<code><nobr>[DS:ESI]</nobr></code> instead of<code><nobr>[DS:SI]</nobr></code>. To access a string in a 16-bit segmentwhen coding in a 32-bit one, the corresponding<code><nobr>a16</nobr></code> prefix can be used.<p>The <code><nobr>a16</nobr></code> and <code><nobr>a32</nobr></code>prefixes can be applied to any instruction in NASM's instruction table, butmost of them can generate all the useful forms without them. The prefixesare necessary only for instructions with implicit addressing:<code><nobr>CMPSx</nobr></code>(<a href="nasmdocb.html#section-B.4.27">section B.4.27</a>),<code><nobr>SCASx</nobr></code>(<a href="nasmdocb.html#section-B.4.286">section B.4.286</a>),<code><nobr>LODSx</nobr></code>(<a href="nasmdocb.html#section-B.4.141">section B.4.141</a>),<code><nobr>STOSx</nobr></code>(<a href="nasmdocb.html#section-B.4.303">section B.4.303</a>),<code><nobr>MOVSx</nobr></code>(<a href="nasmdocb.html#section-B.4.178">section B.4.178</a>),<code><nobr>INSx</nobr></code>(<a href="nasmdocb.html#section-B.4.121">section B.4.121</a>),<code><nobr>OUTSx</nobr></code>(<a href="nasmdocb.html#section-B.4.195">section B.4.195</a>), and<code><nobr>XLATB</nobr></code>(<a href="nasmdocb.html#section-B.4.334">section B.4.334</a>). Also, thevarious push and pop instructions (<code><nobr>PUSHA</nobr></code> and<code><nobr>POPF</nobr></code> as well as the more usual<code><nobr>PUSH</nobr></code> and <code><nobr>POP</nobr></code>) canaccept <code><nobr>a16</nobr></code> or <code><nobr>a32</nobr></code>prefixes to force a particular one of <code><nobr>SP</nobr></code> or<code><nobr>ESP</nobr></code> to be used as a stack pointer, in case thestack segment in use is a different size from the code segment.<p><code><nobr>PUSH</nobr></code> and <code><nobr>POP</nobr></code>, whenapplied to segment registers in 32-bit mode, also have the slightly oddbehaviour that they push and pop 4 bytes at a time, of which the top twoare ignored and the bottom two give the value of the segment register beingmanipulated. To force the 16-bit behaviour of segment-register push and popinstructions, you can use the operand-size prefix<code><nobr>o16</nobr></code>:<p><pre> o16 push ss o16 push ds</pre><p>This code saves a doubleword of stack space by fitting two segmentregisters into the space which would normally be consumed by pushing one.<p>(You can also use the <code><nobr>o32</nobr></code> prefix to force the32-bit behaviour when in 16-bit mode, but this seems less useful.)<p align=center><a href="nasmdo10.html">Next Chapter</a> |<a href="nasmdoc8.html">Previous Chapter</a> |<a href="nasmdoc0.html">Contents</a> |<a href="nasmdoci.html">Index</a></body></html>⌨️ 快捷键说明
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