The Netwide Assembler: NASM

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Chapter 9: Mixing 16 and 32 Bit Code

This chapter tries to cover some of the issues, largely related to unusual forms of addressing and jump instructions, encountered when writing operating system code such as protected-mode initialisation routines, which require code that operates in mixed segment sizes, such as code in a 16-bit segment trying to modify data in a 32-bit one, or jumps between different-size segments.

9.1 Mixed-Size Jumps

The most common form of mixed-size instruction is the one used when writing a 32-bit OS: having done your setup in 16-bit mode, such as loading the kernel, you then have to boot it by switching into protected mode and jumping to the 32-bit kernel start address. In a fully 32-bit OS, this tends to be the only mixed-size instruction you need, since everything before it can be done in pure 16-bit code, and everything after it can be pure 32-bit.

This jump must specify a 48-bit far address, since the target segment is a 32-bit one. However, it must be assembled in a 16-bit segment, so just coding, for example,

        jmp     0x1234:0x56789ABC       ; wrong!

will not work, since the offset part of the address will be truncated to 0x9ABC and the jump will be an ordinary 16-bit far one.

The Linux kernel setup code gets round the inability of as86 to generate the required instruction by coding it manually, using DB instructions. NASM can go one better than that, by actually generating the right instruction itself. Here's how to do it right:

        jmp     dword 0x1234:0x56789ABC         ; right

The DWORD prefix (strictly speaking, it should come after the colon, since it is declaring the offset field to be a doubleword; but NASM will accept either form, since both are unambiguous) forces the offset part to be treated as far, in the assumption that you are deliberately writing a jump from a 16-bit segment to a 32-bit one.

You can do the reverse operation, jumping from a 32-bit segment to a 16-bit one, by means of the WORD prefix:

        jmp     word 0x8765:0x4321      ; 32 to 16 bit

If the WORD prefix is specified in 16-bit mode, or the DWORD prefix in 32-bit mode, they will be ignored, since each is explicitly forcing NASM into a mode it was in anyway.

9.2 Addressing Between Different-Size Segments

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-bit ones. At some point, you will probably end up writing code in a 16-bit segment which has to access data in a 32-bit segment, or vice versa.

If the data you are trying to access in a 32-bit segment lies within the first 64K of the segment, you may be able to get away with using an ordinary 16-bit addressing operation for the purpose; but sooner or later, you will want to do 32-bit addressing from 16-bit mode.

The easiest way to do this is to make sure you use a register for the address, since any effective address containing a 32-bit register is forced to be a 32-bit address. So you can do

        mov     eax,offset_into_32_bit_segment_specified_by_fs 
        mov     dword [fs:eax],0x11223344

This is fine, but slightly cumbersome (since it wastes an instruction and a register) if you already know the precise offset you are aiming at. The x86 architecture does allow 32-bit effective addresses to specify nothing but a 4-byte offset, so why shouldn't NASM be able to generate the best instruction for the purpose?

It can. As in section 9.1, you need only prefix the address with the DWORD keyword, and it will be forced to be a 32-bit address:

        mov     dword [fs:dword my_offset],0x11223344

Also as in section 9.1, NASM is not fussy about whether the DWORD prefix comes before or after the segment override, so arguably a nicer-looking way to code the above instruction is

        mov     dword [dword fs:my_offset],0x11223344

Don't confuse the DWORD prefix outside the square brackets, which controls the size of the data stored at the address, with the one inside the square brackets which controls the length of the address itself. The two can quite easily be different:

        mov     word [dword 0x12345678],0x9ABC

This moves 16 bits of data to an address specified by a 32-bit offset.

You can also specify WORD or DWORD prefixes along with the FAR prefix to indirect far jumps or calls. For example:

        call    dword far [fs:word 0x4321]

This instruction contains an address specified by a 16-bit offset; it loads a 48-bit far pointer from that (16-bit segment and 32-bit offset), and calls that address.

9.3 Other Mixed-Size Instructions

The other way you might want to access data might be using the string instructions (LODSx, STOSx and so on) or the XLATB instruction. These instructions, since they take no parameters, might seem to have no easy way to make them perform 32-bit addressing when assembled in a 16-bit segment.

This is the purpose of NASM's a16 and a32 prefixes. If you are coding LODSB in a 16-bit segment but it is supposed to be accessing a string in a 32-bit segment, you should load the desired address into ESI and then code

        a32     lodsb

The prefix forces the addressing size to 32 bits, meaning that LODSB loads from [DS:ESI] instead of [DS:SI]. To access a string in a 16-bit segment when coding in a 32-bit one, the corresponding a16 prefix can be used.

The a16 and a32 prefixes can be applied to any instruction in NASM's instruction table, but most of them can generate all the useful forms without them. The prefixes are necessary only for instructions with implicit addressing: CMPSx, SCASx, LODSx, STOSx, MOVSx, INSx, OUTSx, and XLATB. Also, the various push and pop instructions (PUSHA and POPF as well as the more usual PUSH and POP) can accept a16 or a32 prefixes to force a particular one of SP or ESP to be used as a stack pointer, in case the stack segment in use is a different size from the code segment.

PUSH and POP, when applied to segment registers in 32-bit mode, also have the slightly odd behaviour that they push and pop 4 bytes at a time, of which the top two are ignored and the bottom two give the value of the segment register being manipulated. To force the 16-bit behaviour of segment-register push and pop instructions, you can use the operand-size prefix o16:

        o16 push    ss 
        o16 push    ds

This code saves a doubleword of stack space by fitting two segment registers into the space which would normally be consumed by pushing one.

(You can also use the o32 prefix to force the 32-bit behaviour when in 16-bit mode, but this seems less useful.)

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