来源：STM32 Cortex®-M4 MCUs and MPUs Programming Manual Rev 10，Page 20

These registers are mutually exclusive bitfields in the 32-bit PSR. The bit assignment is
shown in Figure 3 and Figure 4.

**Figure 3. APSR, IPSR and EPSR bit assignment**

![图 3](imgs/page_20_fig_3.png)

**Figure 4. PSR bit assignment**

![图 4](imgs/page_20_fig_4.png)

Access these registers individually or as a combination of any two or all three registers,
using the register name as an argument to the MSR or MRS instructions. For example:
- Read all of the registers using PSR with the MRS instruction.
- Write to the APSR N, Z, C, V, and Q bits using APSR_nzcvq with the MSR instruction.
The PSR combinations and attributes are:

See the instruction descriptions MRS on page 186 and MSR on page 187 for more
information about how to access the program status registers.

**Table 4. PSR register combinations**

| Register | Type | Combination |
|----------|------|-------------|
| PSR | read-write(1), (2) | APSR, EPSR, and IPSR |
| IEPSR | read-only | EPSR and IPSR |
| IAPSR | read-write(1) | APSR and IPSR |
| EAPSR | read-write(2) | APSR and EPSR |

1. The processor ignores writes to the IPSR bits.
2. Reads of the EPSR bits return zero, and the processor ignores writes to the these bits

|25 24 23 | Reserved | ISR_NUMBER | 31 30 29 28 27 | N Z C V | 0 | Reserved |
|----------|----------|------------|----------------|---------|---|----------|
| 26 | Reserved | | 16 15 | ICI/IT | ICI/IT | T | Q |
| 8 | 19 | 20 | GE[3:0] | Reserved | | |

> 原始图片：imgs/page_20_fig_3.png, imgs/page_20_fig_4.png