(2 32 -2 29
P1 ( PMB)
(2 40 -2 29
Fig. 3.45 Example of logical and physical address spaces of SH-X4
continuously increasing and will soon exceed 2 GB even in an embedded system.
Therefore, we extended the number of physical address bits to 40 bits, which can
define 1-TB address space. The logical address space remained 32 bits, and the
programming model was not changed. Then the binary compatibility was main-
tained. The logical address space extension would require the costly 32-to-64-bit
extensions of register files, integer executions, branch operations, and so on.
Figure 3.45 illustrates an example of the extension. The 32-bit logical address
space is compatible to the predecessors of the SH-X4 in this example. The MMU
translates the logical address to a 32/40-bit physical address by TLB or privileged
mapping buffer (PMB) in 32/40-bit physical address mode, respectively. The TLB
translation is a well-known dynamic method, but the original PMB translation is a
static method to avoid exceptions possible for the TLB translation. Therefore, the
PMB page sizes are larger than that of the TLB in order to cover the PMB area
ef fi ciently.
The logical space is divided into five regions, and the attribute of each region can
be specified as user-mode accessible or inaccessible, translated by TLB or PMB,
and so on. In the example, the P0/U0 region is user-mode accessible and translated
by TLB, the P1 and P2 region are user-mode inaccessible and translated by PMB,
and the P3 region is user-mode inaccessible and translated by TLB. The P4 region
includes a control register area that is mapped on the bottom of physical space so
that the linear physical space is not divided by the control register area.
Data Transfer Unit
High-speed and efficient data transfer is one of the key features for multicore perfor-
mance. The SH-X4 core integrates a DTU for this purpose. A DMAC is conventional
hardware for the data transfer. However, the DTU has some advantage to the DMAC,