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6.36.4 MeP Variable Attributes

The MeP target has a number of addressing modes and busses. The near space spans the standard memory space’s first 16 megabytes (24 bits). The far space spans the entire 32-bit memory space. The based space is a 128-byte region in the memory space that is addressed relative to the $tp register. The tiny space is a 65536-byte region relative to the $gp register. In addition to these memory regions, the MeP target has a separate 16-bit control bus which is specified with cb attributes.

based

Any variable with the based attribute is assigned to the .based section, and is accessed with relative to the $tp register.

tiny

Likewise, the tiny attribute assigned variables to the .tiny section, relative to the $gp register.

near

Variables with the near attribute are assumed to have addresses that fit in a 24-bit addressing mode. This is the default for large variables (-mtiny=4 is the default) but this attribute can override -mtiny= for small variables, or override -ml.

far

Variables with the far attribute are addressed using a full 32-bit address. Since this covers the entire memory space, this allows modules to make no assumptions about where variables might be stored.

io
io (addr)

Variables with the io attribute are used to address memory-mapped peripherals. If an address is specified, the variable is assigned that address, else it is not assigned an address (it is assumed some other module assigns an address). Example:

int timer_count __attribute__((io(0x123)));
cb
cb (addr)

Variables with the cb attribute are used to access the control bus, using special instructions. addr indicates the control bus address. Example:

int cpu_clock __attribute__((cb(0x123)));


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