Inside Macintosh: Mac OS Runtime Architectures /

Chapter 10 - Classic 68K Runtime Architecture

Bypassing MC68000 Addressing Limitations

68K compilers typically generate PC-relative instructions for intrasegment references. This restricts the size of segments to 32 KB because the PC-relative instructions on the MC68000 processor use a 16-bit offset. Similarly, references to addresses expressed as offsets from the address stored in A5 are also limited to 16-bit offsets on the MC68000 processor.

Since references to the jump table are expressed as positive offsets from A5, this effectively limits the size of the jump table to 32 KB. References to global variables are expressed as negative offsets from A5, so the size of the global data area is limited to 32 KB as well.

In the past, the Resource Manager used to limit resources to 32 KB, so 16-bit offsets were guaranteed to be sufficient.

Table 10-1 summarizes existing MPW solutions to these limitations. The sections that follow provide detail on how to implement these solutions. The section "32-Bit Everything" (page 10-17) describes a mechanism that allows you to remove all three limits. Which solution you choose depends on the specific needs of your program.

Note

Other development environments may use different methods to work around the 16-bit addressing limitations.

In general, it is recommended that if you need to remove only one of the limits, you use the solution given for that limit. If you need to remove two or more limits, the 32-bit everything solution is probably your best choice.

Table 10-1 Classic 68K runtime architecture limits and solutions

Problem	Solution	Restrictions and effect on performance
Globals > 32 KB	SC/SCpp -model farData option	No restrictions. Code is bigger. Must link with -model far option.
	Use assembly language for 32-bit references	No restrictions.
Segment > 32 KB	SC/SCpp -bigseg option	Restricted to single-segment C programs running on 68020 and or higher CPU.
	ILink -br 68k option	No restrictions.
	ILink -br 020 option	Program must run on a 68020 or higher.
Jump table > 32 KB	ILink -wrap option	No restrictions. It decreases memory available for global data.
Everything > 32 KB	See the section "32-Bit Everything" (page 10-17).	No restrictions. Code is up to 30% bigger.

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Subtopics

Increasing Global Data Size

Increasing Segment Size

Increasing the Size of the Jump Table

32-Bit Everything