Query: as
OS: minix
Section: 9
Format: Original Unix Latex Style Formatted with HTML and a Horizontal Scroll Bar
This document describes the language accepted by the 80386 assem- bler that is part of the Amsterdam Compiler Kit. Note that only the syntax is described, only a few 386 instructions are shown as examples. The syntax of numbers is the same as in C. The con- stants 32, 040, and 0x20 all represent the same number, but are written in decimal, octal, and hex, respectively. The rules for character constants and strings are also the same as in C. For example, 'a' is a character constant. A typical string is "string". Expressions may be formed with C operators, but must use [ and ] for parentheses. (Normal parentheses are claimed by the operand syntax.) Symbols contain letters and digits, as well as three special characters: dot, tilde, and underscore. The first character may not be a digit or tilde. The names of the 80386 registers are reserved. These are: ~~~al, bl, cl, dl ~~~ah, bh, ch, dh ~~~ax, bx, cx, dx, eax, ebx, ecx, edx ~~~si, di, bp, sp, esi, edi, ebp, esp ~~~cs, ds, ss, es, fs, gs The xx and exx variants of the eight general registers are treated as synonyms by the assembler. Nor- mally "ax" is the 16-bit low half of the 32-bit "eax" register. The assembler determines if a 16 or 32 bit operation is meant solely by looking at the instruction or the instruction prefixes. It is however best to use the proper registers when writing as- sembly to not confuse those who read the code. The last group of 6 segment registers are used for selector + offset mode address- ing, in which the effective address is at a given offset in one of the 6 segments. Names of instructions and pseudo-ops are not reserved. Alphabetic characters in opcodes and pseudo-ops must be in lower case. Commas, blanks, and tabs are separators and can be interspersed freely between tokens, but not within tokens. Commas are only legal between operands. The comment character is !. The rest of the line is ignored. The opcodes are listed be- low. Notes: (1) Different names for the same instruction are separated by /. (2) Square brackets ([]) indicate that 0 or 1 of the enclosed characters can be included. (3) Curly brackets ({}) work similarly, except that one of the enclosed characters must be included. Thus square brackets indicate an option, whereas curly brackets indicate that a choice must be made. mov[b] dest, source ! Move word/byte from source to dest pop dest ! Pop stack push source ! Push stack xchg[b] op1, op2 ! Exchange word/byte xlat ! Translate o16 ! Operate on a 16 bit object instead of 32 bit in[b] source ! Input from source I/O port in[b] ! Input from DX I/O port out[b] dest ! Output to dest I/O port out[b] ! Output to DX I/O port lds reg,source ! Load reg and DS from source les reg,source ! Load reg and ES from source lea reg,source ! Load effect address of source to reg and DS {cdsefg}seg ! Specify seg register for next instruction a16 ! Use 16 bit addressing mode instead of 32 bit lahf ! Load AH from flag register popf ! Pop flags pushf ! Push flags sahf ! Store AH in flag register aaa ! Adjust result of BCD addition add[b] dest,source ! Add adc[b] dest,source ! Add with carry daa ! Decimal Adjust after addition inc[b] dest ! Increment by 1 aas ! Adjust result of BCD subtraction sub[b] dest,source ! Subtract sbb[b] dest,source ! Subtract with borrow from dest das ! Decimal adjust after subtraction dec[b] dest ! Decrement by one neg[b] dest ! Negate cmp[b] dest,source ! Compare aam ! Adjust result of BCD multiply imul[b] source ! Signed multiply mul[b] source ! Unsigned multiply aad ! Adjust AX for BCD division o16 cbw ! Sign extend AL into AH o16 cwd ! Sign extend AX into DX cwde ! Sign extend AX into EAX cdq ! Sign extend EAX into EDX idiv[b] source ! Signed divide div[b] source ! Unsigned divide and[b] dest,source ! Logical and not[b] dest ! Logical not or[b] dest,source ! Logical inclusive or test[b] dest,source ! Logical test xor[b] dest,source ! Logical exclusive or sal[b]/shl[b] dest,CL! Shift logical left sar[b] dest,CL ! Shift arithmetic right shr[b] dest,CL ! Shift logical right rcl[b] dest,CL ! Rotate left, with carry rcr[b] dest,CL ! Rotate right, with carry rol[b] dest,CL ! Rotate left ror[b] dest,CL ! Rotate right cmps[b] ! Compare string element ds:esi with es:edi lods[b] ! Load from ds:esi into AL, AX, or EAX movs[b] ! Move from ds:esi to es:edi rep ! Repeat next instruction until ECX=0 repe/repz ! Repeat next instruction until ECX=0 and ZF=1 repne/repnz ! Repeat next instruction until ECX!=0 and ZF=0 scas[b] ! Compare ds:esi with AL/AX/EAX stos[b] ! Store AL/AX/EAX in es:edi As accepts a number of special jump opcodes that can assemble to instructions with either a byte displacement, which can only reach to targets within -126 to +129 bytes of the branch, or an instruction with a 32-bit displacement. The assembler automati- cally chooses a byte or word displacement instruction. The Eng- lish translation of the opcodes should be obvious, with l(ess) and g(reater) for signed comparisions, and b(elow) and a(bove)*(CQ for unsigned comparisions. There are lots of syn- onyms to allow you to write "jump if not that" instead of "jump if this". The call, jmp, and ret instructions can be either in- trasegment or intersegment. The intersegment versions are indi- cated with the suffix f. jmp[f] dest ! jump to dest (8 or 32-bit displacement) call[f] dest ! call procedure ret[f] ! return from procedure ja/jnbe ! if above/not below or equal (unsigned) jae/jnb/jnc ! if above or equal/not below/not carry (uns.) jb/jnae/jc ! if not above nor equal/below/carry (unsigned) jbe/jna ! if below or equal/not above (unsigned) jg/jnle ! if greater/not less nor equal (signed) jge/jnl ! if greater or equal/not less (signed) jl/jnqe ! if less/not greater nor equal (signed) jle/jgl ! if less or equal/not greater (signed) je/jz ! if equal/zero jne/jnz ! if not equal/not zero jno ! if overflow not set jo ! if overflow set jnp/jpo ! if parity not set/parity odd jp/jpe ! if parity set/parity even jns ! if sign not set js ! if sign set jcxz dest ! jump if ECX = 0 loop dest ! Decrement ECX and jump if CX != 0 loope/loopz dest! Decrement ECX and jump if ECX = 0 and ZF = 1 loopne/loopnz dest! Decrement ECX and jump if ECX != 0 and ZF = 0 int n ! Software interrupt n into ! Interrupt if overflow set iretd ! Return from interrupt clc ! Clear carry flag cld ! Clear direction flag cli ! Clear interrupt enable flag cmc ! Complement carry flag stc ! Set carry flag std ! Set direction flag sti ! Set interrupt enable flag The special symbol . is the location counter and its value is the address of the first byte of the instruction in which the symbol appears and can be used in expressions. There are four different assembly segments: text, rom, data and bss. Segments are de- clared and selected by the .sect pseudo-op. It is customary to declare all segments at the top of an assembly file like this: ~~~.sect .text; .sect .rom; .sect .data; .sect .bss The assembler accepts up to 16 different segments, but expects only four to be used. Anything can in principle be assembled into any segment, but the bss segment may only contain uninitialized data. Note that the . symbol refers to the location in the current segment. There are two types: name and numeric. Name labels consist of a name followed by a colon (:). The numeric labels are single dig- its. The nearest 0: label may be referenced as 0f in the forward direction, or 0b backwards. Each line consists of a single statement. Blank or comment lines are allowed. The most general form of an instruction is ~~~label: opcode operand1, operand2 ! comment The following operators can be used: + - * / & | ^ ~ << (shift left) >> (shift right) - (unary minus). 32-bit integer arithmetic is used. Division produces a truncated quotient. Be- low is a list of the addressing modes supported. Each one is followed by an example. constant mov eax, 123456 direct access mov eax, (counter) register mov eax, esi indirect mov eax, (esi) base + disp. mov eax, 6(ebp) scaled index mov eax, (4*esi) base + index mov eax, (ebp)(2*esi) base + index + disp. mov eax, 10(edi)(1*esi) Any of the constants or symbols may be replacement by expres- sions. Direct access, constants and displacements may be any type of expression. A scaled index with scale 1 may be written without the 1*. The call and jmp instructions can be interpreted as a load into the instruction pointer. call _routine ! Direct, intrasegment call (subloc) ! Indirect, intrasegment call 6(ebp) ! Indirect, intrasegment call ebx ! Direct, intrasegment call (ebx) ! Indirect, intrasegment callf (subloc) ! Indirect, intersegment callf seg:offs ! Direct, intersegment Symbols can acquire values in one of two ways. Using a symbol as a label sets it to . for the current segment with type relocat- able. Alternative, a symbol may be given a name via an assign- ment of the form symbol = expression in which the symbol is assigned the value and type of its arguments. Space can be re- served for bytes, words, and longs using pseudo-ops. They take one or more operands, and for each generate a value whose size is a byte, word (2 bytes) or long (4 bytes). For example: .data1 2, 6 ! allocate 2 bytes initialized to 2 and 6 .data2 3, 0x10 ! allocate 2 words initialized to 3 and 16 .data4 010 ! allocate a longword initialized to 8 .space 40 ! allocates 40 bytes of zeros allocates 50 (decimal) bytes of storage, initializing the first two bytes to 2 and 6, the next two words to 3 and 16, then one longword with value 8 (010 octal), last 40 bytes of zeros. The pseudo-ops .ascii and .asciz take one string argument and generate the ASCII character codes for the letters in the string. The latter auto- matically terminates the string with a null (0) byte. For exam- ple, .ascii "hello" .asciz "world " Sometimes it is necessary to force the next item to begin at a word, longword or even a 16 byte address boundary. The .align pseudo-op zero or more null byte if the current location is a multiple of the argument of .align. Every item assembled goes in one of the four segments: text, rom, data, or bss. By using the .sect pseudo-op with argument .text, .rom, .data or .bss, the programmer can force the next items to go in a particular segment. A symbol can be given global scope by in- cluding it in a .define pseudo-op. Multiple names may be listed, separate by commas. It must be used to export symbols defined in the current program. Names not defined in the current program are treated as "undefined external" automatically, although it is customary to make this explicit with the .extern pseudo-op. The .comm pseudo-op declares storage that can be common to more than one module. There are two arguments: a name and an absolute ex- pression giving the size in bytes of the area named by the sym- bol. The type of the symbol becomes external. The statement can appear in any segment. If you think this has something to do with FORTRAN, you are right. In the kernel directory, there are several assembly code files that are worth inspecting as exam- ples. However, note that these files, are designed to first be run through the C preprocessor. (The very first character is a # to signal this.) Thus they contain numerous constructs that are not pure assembler. For true assembler examples, compile any C program provided with using the -S flag. This will result in an assembly language file with a suffix with the same name as the C source file, but ending with the .s suffix.
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