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CM71-00105-1E FUJITSU MICROELECTRONICS LIMITED 21
CHAPTER 3 PROGRAMMING MODEL
3.3
3.3.4 System Status Register (SSR)
System status register (SSR) is a 4-bit register that indicates the state of the CPU. It lies between bit 31 and
bit 28 of the program status (PS).
Figure 3.3-4 shows the bit configuration of system status register (SSR).
Figure 3.3-4 System Status Register (SSR) Bit Configuration
The contents of each bit are described below.
[bit31] DBG: Debug State Flag
This flag indicates the debugging state during debugging. The flag bit is turned to "1" when the
system shifts to a debug state, and turned to "0" when moving from the debug state with a RETI
instruction. This cannot be rewritten using instructions such as the MOV instruction.
The initial value of the debug state flag (DBG) after a reset is "0".
[bit30] UM: User Mode Flag
This flag indicates the user mode. The flag bit is turned to "1" when the system is shifted to user
mode by the execution of a RETI instruction, and cleared to "0" when shifted to privilege mode with
EIT. Upon execution of the RETI instruction, if bit 30 of the PS value is set to "1", a value returned
from memory, the system shifts to user mode. This cannot be rewritten using instructions such as the
MOV instruction.
The initial value of the user mode flag (UM) after a reset is "0".
[bit29] FPU: FPU presence flag
This flag indicates that the floating point calculation unit (FPU) is installed. The flag bit is set to "1"
if a FPU is installed, and "0" if it is not the case. This bit cannot be rewritten.
Table 3.3-1 FPU presence flag (FPU) in the system status register
[bit28] MPU: MPU presence flag
This flag indicates that the memory protection unit (MPU) is installed. The flag bit is set to "1" if a
MPU is installed, and "0" if it is not the case. This bit cannot be rewritten.
Table 3.3-2 MPU presence flag (MPU) in the system status register
MPUFPUUMDBG
bit31 bit30 bit29 bit28
Initial value
0011
B
flag value Meaning
FPU
0 With FPU (installed)
1 Without FPU (not installed)
flag value Meaning
MPU
0 With MPU (installed)
1 Without MPU (not installed)