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author | lost |
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date | Tue, 27 Jan 2009 05:30:58 +0000 |
parents | 7fbccdd1defb |
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An object file consists of a series of sections each of which contains a list of exported symbols, a list of incomplete references, and a list of "local" symbols which may be used in calculating incomplete references. Each section will obviously also contain the object code. Exported symbols must be completely resolved to an address within the section it is exported from. Each object file starts with a magic number and version number. The magic number is the string "LWOBJ16" for this 16 bit object file format. The only defined version number is currently 0. Thus, the first 8 bytes of the object file are: 4C574F424A313600 Each section has the following items in order: * section name * flags * list of local symbols (and addresses within the section) * list of exported symbols (and addresses within the section) * list of incomplete references along with the expressions to calculate them * the actual object code The section starts with the name of the section with a NUL termination followed by a series of flag bytes terminated by NUL. The following flag bytes are defined: Byte Meaning 00 no more flags 01 section is BSS - no actual code is present Either a NULL section name or end of file indicate the presence of no more sections. Each entry in the exported and local symbols table consists of the symbol (NUL terminated) followed by two bytes which contain the value in big endian order. The end of a symbol table is indicated by a NULL symbol name. Each entry in the incomplete references table consists of an expression followed by a 16 bit offset where the reference goes. Expressions are defined as a series of terms up to an "end of expression" term. Each term consists of a single byte which identifies the type of term (see below) followed by any data required by the term. Then end of the list is flagged by a NULL expression (only an end of expression term). TERMTYPE Meaning 00 end of expression 01 integer (16 bit in big endian order follows) 02 external symbol reference (NUL term symbol) 03 local symbol reference (NUL term symbol) 04 operator (1 byte operator number - see below) 05 section base address reference External references are resolved using other object files while local references are resolved using the local symbol table(s) from this file. This allows local symbols that are not exported to have the same names as exported symbols or external references. The operator numbers are: NUM OP 01 + (plus) 02 - (minus) 03 * (times) 04 / (divide) 05 % (modulus) 06 \ (integer division) 07 bitwise and 08 bitwise or 09 bitwise xor 0A boolean and 0B boolean or 0C - (unary negation, 2's complement) 0D ^ (unary 1's complement) An expression is represented in a postfix manner with both operands for binary operators preceding the operator and the single operand for unary operators preceding the operator.