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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.
basedAny variable with the based attribute is assigned to the
.based section, and is accessed with relative to the
$tp register.
tinyLikewise, the tiny attribute assigned variables to the
.tiny section, relative to the $gp register.
nearVariables 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.
farVariables 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.
ioio (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)));
cbcb (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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