The Cortex-M3 processor PM0056
20/155 Doc ID 15491 Rev 4
Execution program status register
The EPSR contains the Thumb state bit, and the execution state bits for either the:
● If-Then (IT) instruction
● Interruptible-Continuable Instruction (ICI) field for an interrupted load multiple or store
multiple instruction.
See the register summary in Table 2 on page 15 for the EPSR attributes. The bit
assignments are:
Attempts to read the EPSR directly through application software using the MSR instruction
always return zero. Attempts to write the EPSR using the MSR instruction in application
software are ignored. Fault handlers can examine EPSR value in the stacked PSR to
indicate the operation that is at fault. See Section 2.3.7: Exception entry and return on
page 38
Interruptible-continuable instructions
When an interrupt occurs during the execution of an LDM or STM instruction, the processor:
● Stops the load multiple or store multiple instruction operation temporarily
● Stores the next register operand in the multiple operation to EPSR bits[15:12].
After servicing the interrupt, the processor:
● Returns to the register pointed to by bits[15:12]
● Resumes execution of the multiple load or store instruction.
When the EPSR holds ICI execution state, bits[26:25,11:10] are zero.
If-Then block
The If-Then block contains up to four instructions following a 16-bit IT instruction. Each
instruction in the block is conditional. The conditions for the instructions are either all the
same, or some can be the inverse of others. See IT on page 95 for more information.
Exception mask registers
The exception mask registers disable the handling of exceptions by the processor. Disable
exceptions where they might impact on timing critical tasks.
To access the exception mask registers use the MSR and MRS instructions, or the CPS
instruction to change the value of PRIMASK or FAULTMASK. See MRS on page 101, MSR
on page 102, and CPS on page 99 for more information.
Table 6. EPSR bit definitions
Bits Description
Bits 31:27 Reserved.
Bits 26:25, 15:10 ICI: Interruptible-continuable instruction bits
See Interruptible-continuable instructions on page 20.
Bits 26:25, 15:10 IT: Indicates the execution state bits of the IT instruction, see
IT on page 95.
Bit 24 Always set to 1.
Bits 23:16 Reserved.
Bits 9:0] Reserved.