Processor Programming (Continued)
3.4 ADDRESS SPACES
The GXm processor can directly address either memory
or I/O space. Figure 3-2 illustrates the range of addresses
available for memory address space and I/O address
space. For the CPU, the addresses for physical memory
range between 00000000h and FFFFFFFFh (4 GBytes).
The accessible I/O addresses space ranges between
00000000h and 0000FFFFh (64 KB). The CPU does not
use coprocessor communication space in upper I/O space
between 800000F8h and 800000FFh as do the 386-style
CPUs. The I/O locations 22h and 23h are used for GXm
processor configuration register access.
I/O accesses to port address range 3B0h through 3DFh
can be trapped to SMI by the CPU if this option is enabled
in the BC_XMAP_1 register (see SMIB, SMIC, and SMID
bits in Table 4-10 on page 101). Figure 3-2 illustrates the
I/O address space.
3.4.2 Memory Address Space
The processor directly addresses up to 4 GB of physical
memory even though the memory controller addresses
only 1 GB of DRAM. Memory address space is accessed
as bytes, words (16 bits) or DWORDs (32 bits). Words
and DWORDs are stored in consecutive memory bytes
with the low-order byte located in the lowest address. The
physical address of a word or DWORD is the byte address
of the low-order byte.
3.4.1 I/O Address Space
The CPU I/O address space is accessed using IN and
OUT instructions to addresses referred to as “ports.” The
accessible I/O address space is 64 KB and can be
accessed as 8-bit, 16-bit or 32-bit ports.
The processor allows memory to be addressed using nine
different addressing modes. These addressing modes are
used to calculate an offset address, often referred to as an
effective address. Depending on the operating mode of
the CPU, the offset is then combined, using memory man-
agement mechanisms, into a physical address that is
applied to the physical memory devices.
The GXm processor configuration registers reside within
the I/O address space at port addresses 22h and 23h and
are accessed using the standard IN and OUT instructions.
The configuration registers are modified by writing the
index of the configuration register to port 22h, and then
transferring the data through port 23h. Accesses to the
on-chip configuration registers do not generate external
I/O cycles. However, each operation on port 23h must be
preceded by a write to port 22h with a valid index value.
Otherwise, subsequent port 23h operations will communi-
cate through the I/O port to produce external I/O cycles with-
out modifying the on-chip configuration registers. Write
operations to port 22h outside of the CPU index range
(C0h-CFh and FEh-FFh) result in external I/O cycles and
do not affect the on-chip configuration registers. Reading
port 22h generates external I/O cycles.
Memory management mechanisms consist of segmenta-
tion and paging. Segmentation allows each program to
use several independent, protected address spaces. Pag-
ing translates a logical address into a physical address
using translation lookup tables. Virtual memory is often
implemented using paging. Either or both of these mecha-
nisms can be used for management of the GXm proces-
sor memory address space.
Accessible
Programmed
I/O Space
Physical
Memory Space
FFFFFFFFh
FFFFFFFFh
Not
Accessible
Physical Memory
4 GB
CPU General
Configuration
Register I/O
0000FFFFh
Space
64 KB
00000023h
00000022h
00000000h
00000000h
Figure 3-2. Memory and I/O Address Spaces
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