NASM

Effective Addresses

An effective address is any operand to an instruction which references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in square brackets. For example: wordvar dw 123 mov ax,[wordvar] mov ax,[wordvar+1] mov ax,[es:wordvar+bx] Anything not conforming to this simple system is not a valid memory reference in NASM, for example es:wordvar[bx]. More complicated effective addresses, such as those involving more than one register, work in exactly the same way: mov eax,[ebx*2+ecx+offset] mov ax,[bp+di+8] NASM is capable of doing algebra on these effective addresses, so that things which don't necessarily look legal are perfectly all right: mov eax,[ebx*5] ; assembles as [ebx*4+ebx] mov eax,[label1*2-label2] ; ie [label1+(label1-label2)] Some forms of effective address have more than one assembled form; in most such cases NASM will generate the smallest form it can. For example, there are distinct assembled forms for the 32-bit effective addresses [eax*2+0] and [eax+eax], and NASM will generally generate the latter on the grounds that the former requires four bytes to store a zero offset. NASM has a hinting mechanism which will cause [eax+ebx] and [ebx+eax] to generate different opcodes; this is occasionally useful because [esi+ebp] and [ebp+esi] have different default segment registers. However, you can force NASM to generate an effective address in a particular form by the use of the keywords BYTE, WORD, DWORD and NOSPLIT. If you need [eax+3] to be assembled using a double-word offset field instead of the one byte NASM will normally generate, you can code [dword eax+3]. Similarly, you can force NASM to use a byte offset for a small value which it hasn't seen on the first pass (see section 3.8 for an example of such a code fragment) by using [byte eax+offset]. As special cases, [byte eax] will code [eax+0] with a byte offset of zero, and [dword eax] will code it with a double-word offset of zero. The normal form, [eax], will be coded with no offset field. The form described in the previous paragraph is also useful if you are trying to access data in a 32-bit segment from within 16 bit code. For more information on this see the section on mixed-size addressing (section 10.2). In particular, if you need to access data with a known offset that is larger than will fit in a 16-bit value, if you don't specify that it is a dword offset, nasm will cause the high word of the offset to be lost. Similarly, NASM will split [eax*2] into [eax+eax] because that allows the offset field to be absent and space to be saved; in fact, it will also split [eax*2+offset] into [eax+eax+offset]. You can combat this behaviour by the use of the NOSPLIT keyword: [nosplit eax*2] will force [eax*2+0] to be generated literally. In 64-bit mode, NASM will by default generate absolute addresses. The REL keyword makes it produce RIP-relative addresses. Since this is frequently the normally desired behaviour, see the DEFAULT directive (section 6.2). The keyword ABS overrides REL.

DB and Friends: Declaring Initialized Data

DB, DW, DD, DQ, DT, DO and DY are used, much as in MASM, to declare initialized data in the output file. They can be invoked in a wide range of ways: db 0x55 ; just the byte 0x55 db 0x55,0x56,0x57 ; three bytes in succession db 'a',0x55 ; character constants are OK db 'hello',13,10,'$' ; so are string constants dw 0x1234 ; 0x34 0x12 dw 'a' ; 0x61 0x00 (it's just a number) dw 'ab' ; 0x61 0x62 (character constant) dw 'abc' ; 0x61 0x62 0x63 0x00 (string) dd 0x12345678 ; 0x78 0x56 0x34 0x12 dd 1.234567e20 ; floating-point constant dq 0x123456789abcdef0 ; eight byte constant dq 1.234567e20 ; double-precision float dt 1.234567e20 ; extended-precision float DT, DO and DY do not accept numeric constants as operands.

EQU: Defining Constants

EQU defines a symbol to a given constant value: when EQU is used, the source line must contain a label. The action of EQU is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example, message db 'hello, world' msglen equ $-message defines msglen to be the constant 12. msglen may not then be redefined later. This is not a preprocessor definition either: the value of msglen is evaluated once, using the value of $ (see section 3.5 for an explanation of $) at the point of definition, rather than being evaluated wherever it is referenced and using the value of $ at the point of reference.

INCBIN: Including External Binary Files

INCBIN is borrowed from the old Amiga assembler DevPac: it includes a binary file verbatim into the output file. This can be handy for (for example) including graphics and sound data directly into a game executable file. It can be called in one of these three ways: incbin "file.dat" ; include the whole file incbin "file.dat",1024 ; skip the first 1024 bytes incbin "file.dat",1024,512 ; skip the first 1024, and ; actually include at most 512 INCBIN is both a directive and a standard macro; the standard macro version searches for the file in the include file search path and adds the file to the dependency lists. This macro can be overridden if desired.

RESB and Friends: Declaring Uninitialized Data

RESB, RESW, RESD, RESQ, REST, RESO and RESY are designed to be used in the BSS section of a module: they declare uninitialized storage space. Each takes a single operand, which is the number of bytes, words, doublewords or whatever to reserve. As stated in section 2.2.7, NASM does not support the MASM/TASM syntax of reserving uninitialized space by writing DW ? or similar things: this is what it does instead. The operand to a RESB-type pseudo-instruction is a critical expression: see section 3.8. For example: buffer: resb 64 ; reserve 64 bytes wordvar: resw 1 ; reserve a word realarray resq 10 ; array of ten reals ymmval: resy 1 ; one YMM register

TIMES: Repeating Instructions or Data

The TIMES prefix causes the instruction to be assembled multiple times. This is partly present as NASM's equivalent of the DUP syntax supported by MASM-compatible assemblers, in that you can code zerobuf: times 64 db 0 or similar things; but TIMES is more versatile than that. The argument to TIMES is not just a numeric constant, but a numeric expression, so you can do things like buffer: db 'hello, world' times 64-$+buffer db ' ' which will store exactly enough spaces to make the total length of buffer up to 64. Finally, TIMES can be applied to ordinary instructions, so you can code trivial unrolled loops in it: times 100 movsb Note that there is no effective difference between times 100 resb 1 and resb 100, except that the latter will be assembled about 100 times faster due to the internal structure of the assembler. The operand to TIMES is a critical expression (section 3.8). Note also that TIMES can't be applied to macros: the reason for this is that TIMES is processed after the macro phase, which allows the argument to TIMES to contain expressions such as 64-$+buffer as above. To repeat more than one line of code, or a complex macro, use the preprocessor %rep directive.