Internal and External Addressing Modes

An address mode is applied to a memory segment as a part of the ROM image or at the executing (or logical) address of the segment. Following address modes can be applied to a memory segment:

Internal-the segment executes from an address not specified at link time.
External- the segment must execute from the address specified at the link time.

Consider an example where the segment .foo is a part of ROM Image and will be copied to a RAM location. The link time addresses are:

ROM = 0x00100000
RAM = 0x00002000
Note: Both the link time addresses can be external or internal.

Also assume that the real time (physical) ROM address is 0x00200000 instead of the link time specified address 0x00100000. Possible addr_mode Scenarios lists the possible address mode scenarios.

Table 1. Possible addr_mode Scenarios
Scenario	ROM addr_mode	RAM addr_mode	Description
A	internal	external	Runtime correctly figures out that the ROM address is 0x00200000 and copies it to 0x00002000
B	internal	internal	Runtime correctly figures out that the ROM address is 0x00200000 and copies it to 0x00102000
C	external	external	Runtime incorrectly assumes that the ROM address is 0x00100000 and copies it to 0x00002000
D	external	internal	Runtime incorrectly assumes that the ROM address is 0x00100000 and copies it to 0x00102000

In the above possible scenarios only A and B are correct. The difference between scenario A and B is that in A, the executing (logical) address of .foo is absolute and that in B, the executing (logical) address of.foo is relative.

Scenario C and D are possible if .foo is flashed to ROM at its correct ROM address and all other segments are at an offset from their link time ROM addresses.

Note: .init segment determines the correct address of an application. If .init is at its link time ROM address, then all the segments in the application will be treated as external.