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Inside Macintosh: Mac OS Runtime Architectures /

Chapter 10 - Classic 68K Runtime Architecture / Bypassing MC68000 Addressing Limitations

Increasing Segment Size

There are two methods for increasing segment size:

• You can use the -bigseg compiler option. This causes function calls within the same segment to be encoded with the BSR.L instruction (available on 68020 or higher CPUs), which is a PC-relative instruction with a 32-bit offset. This solution is right for single-code segments like command extensions (type 'XCMD') written in C. It does not work on 68000 machines.
• You can use the -br 68k or -br 020 option of the ILink command. ILink then inserts small assembly-language modules called branch islands that transmit calls between two distant modules. The original call is modified to be a JSR instruction to the branch island, and the latter contains instructions to branch to the desired target.

Note

If the program you are writing is intended to run on a 68020 or higher CPU, you can use the -br 020 option. This reduces code size and improves execution speed.

Creating branch islands solves intrasegment reference problems, but is not a complete solution in the case where a routine located beyond the 32 KB limit is externally referenced. Figure 10-7 shows two segments, one of which is larger than 32 KB.

Figure 10-7 Branch islands and intersegment references

As you can see, the only reference that cannot be resolved is that to procedure B if it is made through the jump table. The ILink tool automatically tries to place externally referenced routines in the first 32 KB of a segment, but if this is not possible, it generates a linker error. In such cases, you should resegment your code or build your program with the -model far (32-bit everything) option.