SPRU513Z August 2001 – October 2023 SM320F28335-EP
Sometimes an expression contains more than one relocatable symbol, or cannot be evaluated at assembly time. In this case, the assembler encodes the entire expression in the object file. After determining the addresses of the symbols, the linker computes the value of the expression.
If the value of an expression is larger, in bits, than the space reserved for it, you will receive an error message from the linker.
Each section in an object module has a table of relocation entries. The table contains one relocation entry for each relocatable reference in the section. The linker usually removes relocation entries after it uses them. This prevents the output file from being relocated again (if it is relinked or when it is loaded). A file that contains no relocation entries is an absolute file (all its addresses are absolute addresses, which are addresses known at assembly time). If you want the linker to retain relocation entries, invoke the linker with the --relocatable option (see Section 8.4.3.2).
In the example in Section 2.7, both symbols X and Y are relocatable. Y is defined in the .text section of this module; X is defined in another module. When the code is assembled, X has a value of 0 (the assembler assumes all undefined external symbols have values of 0), and Y has a value of 4 (relative to address 0 in the .text section). The assembler generates two relocation entries: one for X and one for Y. The reference to X is an external reference (indicated by the ! character in the listing). The reference to Y is to an internally defined relocatable symbol (indicated by the ' character in the listing).
After the code is linked, suppose that X is relocated to address 0x7100. Suppose also that the .text section is relocated to begin at address 0x7200; Y now has a relocated value of 0x7204. The linker uses the two relocation entries to patch the two references in the object code:
0080' LC Y |
becomes | 0080' |
0004 |
7204 |
|
28A1! MOV AR1,#X |
becomes | 28A1! |
0000 |
7100 |
Sometimes an expression contains more than one relocatable symbol, or cannot be evaluated at assembly time. In this case, the assembler encodes the entire expression in the object file. After determining the addresses of the symbols, the linker computes the value of the expression as shown in the following example.
1 .global sym1, sym2
2
3 00000000 FF20% MOV ACC, #(sym2-sym1)
00000001 0000The symbols sym1 and sym2 are both externally defined. Therefore, the assembler cannot evaluate the expression sym2 - sym1, so it encodes the expression in the object file. The '%' listing character indicates a relocation expression. Suppose the linker relocates sym2 to 300h and sym1 to 200h. Then the linker computes the value of the expression to be 300h - 200h = 100h. Thus the MOV instruction is patched to:
00000000 FF20 MOV ACC, #(sym2-sym1)
00000001 0100