Does Bit Clearing The Flags In Status Register Do Anything
Status Register
Control/status register (address: 64h) – used either to decide the condition of the keyboard (when a value is read from the register) or to gear up the keyboard (when a value is written to the register).
From: Computer Busses , 2000
Cortex-M3 Basics
Joseph Yiu , in The Definitive Guide to the ARM Cortex-M3 (Second Edition), 2010
iii.ii.1 Programme Status Registers
The PSRs are subdivided into three condition registers:
- •
-
Application Program Condition register (APSR)
- •
-
Interrupt Program Condition register (IPSR)
- •
-
Execution Plan Status annals (EPSR)
The iii PSRs tin can be accessed together or separately using the special annals access instructions MSR and MRS. When they are accessed every bit a collective item, the name xPSR is used.
You lot tin read the PSRs using the MRS instruction. Yous can also change the APSR using the MSR instruction, but EPSR and IPSR are read-merely. For instance:
MRS r0, APSR ; Read Flag state into R0
MRS r0, IPSR ; Read Exception/Interrupt state
MRS r0, EPSR ; Read Execution state
MSR APSR, r0 ; Write Flag land
In ARM assembler, when accessing xPSR (all 3 PSRs as one), the symbol PSR is used:
MRS r0, PSR ; Read the combined program status word
MSR PSR, r0 ; Write combined program state give-and-take
The descriptions for the bit fields in PSR are shown in Table 3.1.
Table 3.1. Bit Fields in Cortex-M3 Plan Status Registers
| Bit | Description |
|---|---|
| North | Negative |
| Z | Zip |
| C | Carry/infringe |
| Five | Overflow |
| Q | Sticky saturation flag |
| ICI/IT | Interrupt-Continuable Pedagogy (ICI) $.25, IF-THEN instruction status bit |
| T | Thumb state, always 1; trying to clear this bit will cause a error exception |
| Exception number | Indicates which exception the processor is handling |
If you compare this with the Electric current Program Condition register (CPSR) in ARM7, y'all might detect that some bit fields that were used in ARM7 are gone. The Way (M) flake field is gone because the Cortex-M3 does not have the operation style as defined in ARM7. Thumb-chip (T) is moved to chip 24. Interrupt condition (I and F) $.25 are replaced by the new interrupt mask registers (PRIMASKs), which are separated from PSR. For comparison, the CPSR in traditional ARM processors is shown in Figure 3.v.
FIGURE 3.5. Current Program Condition Registers in Traditional ARM Processors.
Read full affiliate
URL:
https://www.sciencedirect.com/science/article/pii/B9781856179638000065
Computer Compages
William J. Buchanan BSc, CEng, PhD , in Software Evolution for Engineers, 1997
fourteen.4.3 Status registers
Status registers are used to test for various weather in an operation, such as 'is the result negative', 'is the upshot zippo', and so on. The 2 status registers take 16 bits and are chosen the instruction pointer (IP) and the flag register (F):
- •
-
IP, which is the educational activity pointer. The IP register contains the accost of the next instruction of the program.
- •
-
Flag register. The flag annals holds a collection of sixteen dissimilar conditions. Table fourteen.one outlines the most used flags.
Table 14.1. Processor flags
Scrap Flag position Name Description C 0 Set on behave Contains the conduct from the most significant bit (left paw scrap) post-obit a shift, rotate or arithmetic operation. A 4 Attack i/2 carry S 7 Set on negative issue Contains the sign of an arithmetic operation (0 for positive, one for negative). Z 6 Ready on zero upshot Contains results of final arithmetic or compare result (0 for non-null, ane for nada). 0 11 Assail overflow Indicates that an overflow has occurred in the virtually significant fleck from an arithmetics performance. P 2 Prepare on even parity D ten Management I 9 Interrupt enable Indicates whether the interrupt has been disabled. T eight Trap
Read total chapter
URL:
https://www.sciencedirect.com/scientific discipline/article/pii/B9780340700143500591
Education ready
Joseph Yiu , in Definitive Guide to Arm® Cortex®-M23 and Cortex-M33 Processors, 2021
5.14.4 Conditional branch
Conditional branches are executed conditionally based on the current value in the APSR (N, Z, C, and 5 flags, as shown in Table five.56).
Table five.56. Flags (status bits) in the APSR which tin be used for controlling conditional branches.
| Flag | PSR flake | Clarification |
|---|---|---|
| Northward | 31 | Negative flag (last functioning result is a negative value) |
| Z | thirty | Nothing (last operation effect returned a zero value. For example, the compare of ii registers with identical values.) |
| C | 29 | Carry flag.
|
| V | 28 | Overflow (the last performance resulted in an overflow state of affairs) |
APSR flags can be affected by:
- •
-
Almost 16-bit data processing instructions.
- •
-
32-bit (Thumb-2) information processing instructions with the Southward suffix; for example, ADDS.W.
- •
-
Compare (e.grand., CMP) and Test (due east.1000., TST, TEQ) instructions.
- •
-
An educational activity that writes to APSR/xPSR directly.
- •
-
Unstacking operations at the end of an exception/interrupt service.
For Armv8-M Mainline processors, in that location is another APSR flag bit at bit 27—called the Q flag. It is for saturating arithmetic operations, but is not used for conditional branches.
The status blazon of a conditional branch instruction is indicated by a suffix, which is listed in Table v.58. These suffixes are also used for conditional execution operations (see Section 5.14.vi). Conditional branch instructions (Table 5.57, where <cond> is one of the condition suffixes) are available in 16-bit and 32-bit versions. The 16-bit and 32-bit versions have different branch ranges, with the Armv8-Yard Baseline but supporting the 16-flake version of the conditional branch instructions.
Table five.57. Instructions for provisional branch.
| Instruction | Description | Brake |
|---|---|---|
| A 16-flake version of the conditional branch. <cond> is one of the status suffixes in Tabular array 5.58 | It has a co-operative range of −256 bytes to +254 bytes |
| A 32-flake version of the conditional branch Note: this can exist written as "B <cond> label" if the assembler automatically selects the 32-scrap version when the offset is > 2KB. <cond> is i of the condition suffixes in Table 5.58 | Information technology has a branch range of +/−1MB. Not supported in Armv8-Thou Baseline |
The <cond> is one of the fourteen possible condition suffixes listed in Tabular array v.58.
Table 5.58. Suffixes for conditional branches and provisional executions.
| Suffix | Co-operative status | Flags (APSR) |
|---|---|---|
| EQ | Equal | Z flag is set |
| NE | Non equal | Z flag is cleared |
| CS/HS | Carry set/unsigned higher or aforementioned | C flag is set |
| CC/LO | Carry clear/unsigned lower | C flag is cleared |
| MI | Minus/negative | N flag is set (minus) |
| PL | Plus/positive or zero | N flag is cleared |
| VS | Overflow | V flag is set |
| VC | No overflow | V flag is cleared |
| HI | Unsigned higher | C flag is fix and Z is cleared |
| LS | Unsigned lower or same | C flag is cleared or Z is fix |
| GE | Signed greater than or equal | N flag is ready and V flag is set up, or N flag is cleared and V flag is cleared (N == V) |
| LT | Signed less than | N flag is set and Five flag is cleared, or N flag is cleared and 5 flag is set (N != 5) |
| GT | Signed greater than | Z flag is cleared, and either both the N flag and V flag are ready, or both the Northward flag and V flag are cleared (Z == 0 and N == V) |
| LE | Signed less than or equal | Z flag is prepare, or one of the followings: either the N flag is set and the V flag is cleared, or the Northward flag is cleared and the V flag is set (Z == ane or Due north != V) |
An instance of using a conditional branch educational activity is shown in Fig. five.sixteen. The operation in the diagram selects a new value for R3 based on the value in R0.
Fig. 5.sixteen. Simple conditional co-operative example.
The program catamenia in Fig. v.xvi tin be implemented using provisional branch and regular co-operative instructions every bit follows:
-
CMP R0, #1 ; compare R0 to 1
-
BEQ p2 ; if Equal, then go to p2
-
MOVS R3, #1 ; R3 = 1
-
B p3 ; go to p3
-
p2 ; label p2
-
MOVS R3, #2
-
p3 ; label p3
-
... ; other subsequence operations
Read full chapter
URL:
https://www.sciencedirect.com/science/article/pii/B9780128207352000056
Architecture
Joseph Yiu , in Definitive Guide to Arm® Cortex®-M23 and Cortex-M33 Processors, 2021
Program Condition Annals (PSR)
The Program Condition Register is 32-bit and can be subdivided into:
- •
-
Application PSR—contains various "ALU flags" which are required for conditional branches and instruction operations that need special flags (e.k., add together with bear flag).
- •
-
Execution PSR—contains execution state information.
- •
-
Interrupt PSR—contains current interrupt/exception country data.
These three registers can be accessed as ane combined register, sometimes referred to as "xPSR" in some documentation (e.g., Armv8-M Architecture Reference Manual), as shown in Fig. four.seven. In programming, the symbol PSR is used when accessing the whole PSR. For instance,
Fig. 4.7. Program Status Registers—APSR, IPSR, EPSR, and xPSR.
-
MRS r0, PSR; Read the combined program status give-and-take
-
MSR PSR, r0; Write combined program state word
You lot can as well access each PSR individually. For example:
-
MRS r0, APSR; Read flag states into register r0
-
MRS r0, IPSR; Read exception/interrupt states into register r0
-
MSR APSR, r0; Write flag state
Table 4.2 shows the register symbols available for accessing xPSR.
Tabular array 4.two. Valid symbols of xPSR for programming.
| Symbol | Description |
|---|---|
| APSR | Application PSR only |
| EPSR | Execution PSR only |
| IPSR | Interrupt PSR but |
| IAPSR | Combination of APSR and IPSR |
| EAPSR | Combination of APSR and EPSR |
| IEPSR | Combination of IPSR and EPSR |
| PSR | Combination of APSR, IPSR, and EPSR |
Please note, at that place are some restrictions:
- •
-
The EPSR cannot exist accessed by software lawmaking direct past using the MRS (it is read as aught) or the MSR. Only information technology is visible during exception sequences (when xPSR is saved and restored into the stack), and is visible from debug tools.
- •
-
The IPSR is read-just and cannot be changed using a MSR instruction.
Table 4.3 below lists the definitions of the bit fields in the PSRs.
Table 4.3. Bit fields in the Programme Status Registers.
| Bit | Description |
|---|---|
| Northward | Negative flag |
| Z | Zero flag |
| C | Conduct (or NOT borrow) flag |
| V | Overflow flag |
| Q | Glutinous saturation flag (Available in Armv8-Thou Mainline and Arvmv7-Thou. Not available in Armv8-1000 Baseline and Armv6-Grand) |
| GE[3:0] | Greater-Than or Equal flags for each byte lane (available in Armv8-M Mainline and Arvmv7-Thousand when the DSP extension is implemented). This is updated by a range of instructions in the DSP extension and can be utilized by the SEL (select) instruction. |
| ICI/IT | Interrupt-Continuable Education (ICI) bits and IF-So pedagogy status bit for provisional execution (Available in Armv8-K Mainline and Armv7-M. Not available in Armv8-M Baseline and Armv6-K) |
| T | Thumb country, always 1 for normal operations. Attempts to articulate this bit will cause a fault exception. |
| Exception Number | Indicates which exception/interrupt service the processor is handling. |
Note: If TrustZone is implemented and if a Secure exception handlers calls a Non-secure function, the value of IPSR is set to one during the function call to mask the identity of the Secure exception service.
Compared to the Armv6-M architecture, Armv8-K expanded the width of the exception number field in the Program Condition Register to 9 bits. This enables the Armv8-M processor to back up more interrupts: The Cortex-M23 processor supports up to 240 interrupts compared to 32 interrupts in the Cortex-M0 and Cortex-M0+ processors (the exception number field is 5-bit wide in these processors).
The other fields in the xPSR are the same as those in the Armv6-M or Armv7-M architectures. Equally in the by, some of the flake fields are not available in Armv8-M Baseline (i.e., the Cortex-M23 processor).
Fig. iv.8 shows the PSR flake fields in various Arm architectures. Delight annotation, PSRs in Cortex-One thousand processors are unlike from archetype processors like the Arm7TDMI™. For example, archetype Arm processors have Fashion (Yard) bits, and the T scrap is in bit 5 rather than bit 24. In addition, the interrupt masking $.25 (I and F) in classic Arm processors is separated into new interrupt masking registers (e.g., PRIMASK, FAULTMASK).
Fig. 4.viii. Comparison of Program Status Registers in different Arm architectures.
Detailed behavior of the APSR is covered in Section 4.two.3.
The ICI/Information technology $.25 are available in Armv7-M and Armv8-M Mainline and serves two purposes:
- •
-
During the execution of an Information technology (IF-And then) education block, these bits (It) hold the provisional execution data.
- •
-
During the execution of a multiple load/store instruction, these $.25 (ICI) hold the current progress status of the educational activity.
The ICI/Information technology chip fields overlap as well-nigh of the instance programme codes do not employ the two features at the same time. When an exception occurs, the ICI/It state is saved as part of the automatic stacking functioning(run across Section 8.4.3—the ICI/It bits are inside the stacked xPSR). After the interrupt is served, the execution of the interrupted code is resumed using the restored ICI/Information technology bits.
More information on ICI/Information technology bits is covered in Section nine.6.2.
Read full chapter
URL:
https://www.sciencedirect.com/science/article/pii/B9780128207352000044
Cortex-M Architecture
Trevor Martin , in The Designer's Guide to the Cortex-M Processor Family (2nd Edition), 2016
Program Status Register
The PSR as its name implies contains all the CPU status flags (Fig. 3.three).
Effigy 3.iii. The Programme Condition Register contains several groups of CPU flags. These include the condition codes (NZCVQ), Interrupt Continuable Instruction (ICI) status bits, If And then flag, and current exception number.
The PSR has a number of alias fields that are masked versions of the full register. The three alias registers are the Application PSR, Interrupt PSR, and the Execution PSR. Each of these alias registers contains a subset of the full register flags and tin can be used as a shortcut if y'all need to access role of the PSR. The PSR is generally referred to as the xPSR to signal the total annals rather than any of the alias subsets (Fig. 3.four).
Figure 3.4. The Program Status Register has three allonym registers which provide access to specific subregions or the Program Status Register. Hence, the generic name for the program status annals is xPSR.
In a normal application program, your code will not make explicit admission to the xPSR or any of its alias registers. Any use of the xPSR will be made by compiler generated code. Every bit a programmer y'all need to have an awareness of the xPSR and the flags contained in it.
The most significant four bits of the xPSR are the condition code bits, Negative, Zero, Carry, and oVerflow. These will be prepare and cleared depending on the results of a data processing instruction. The result of Pollex-2 data processing instructions can gear up or clear these flags. However, updating these flags is optional.
SUB R8, R6, #240 Perform a subtraction and do not update the condition lawmaking flags
SUBS R8, R6, #240 Perform a subtraction and update the condition lawmaking flags
This allows the compiler to perform an instruction that updates the condition lawmaking flags, and then perform some additional instructions that do not modify the flags, and then perform a conditional branch on the state of the xPSR condition codes. Following the iv condition code flags is a further instruction flag the Q bit.
Read total affiliate
URL:
https://www.sciencedirect.com/science/article/pii/B9780081006290000037
Architecture
Joseph Yiu , in The Definitive Guide to ARM® CORTEX®-M3 and CORTEX®-M4 Processors (Third Edition), 2014
4.iii Behavior of the application programme status annals (APSR)
The APSR contains several groups of status flags:
- •
-
Condition flags for integer operations (N-Z-C-V bits)
- •
-
Status flags for saturation arithmetic (Q chip)
- •
-
Condition flags for SIMD operations (GE bits)
4.3.1 Integer status flags
The integer status flags are very similar to ALU status flags in many other processor architectures. These flags are affected past full general data processing instructions, and are essential for controlling conditional branches and conditional executions. In addition, one of the APSR flags, the C (Carry) chip, can also be used in add and subtract operations.
There are four integer flags in the Cortex®-One thousand processors, shown in Table 4.vi. A few examples of the ALU flag results are shown in Table 4.7.
Table 4.half-dozen. ALU Flags on the Cortex-M Processors
| Flag | Descriptions |
|---|---|
| N (flake 31) | Set to bit[31] of the result of the executed instruction. When it is "1," the result has a negative value (when interpreted equally a signed integer). When it is "0," the result has a positive value or equal zero. |
| Z (bit 30) | Set up to "1" if the result of the executed didactics is null. It can likewise be set to "1" afterward a compare didactics is executed if the two values are the same. |
| C (bit 29) | Deport flag of the consequence. For unsigned addition, this bit is set to "1" if an unsigned overflow occurred. For unsigned subtract operations, this bit is the inverse of the borrow output status. This fleck is also updated by shift and rotate operations. |
| V (fleck 28) | Overflow of the upshot. For signed addition or subtraction, this bit is set to "i" if a signed overflow occurred. |
Table 4.seven. ALU Flags Instance
| Operation | Results, Flags |
|---|---|
| 0x70000000 + 0x70000000 | Result = 0xE0000000, N= 1, Z=0, C = 0, V = ane |
| 0x90000000 + 0x90000000 | Result = 0x20000000, Northward= 0, Z=0, C = 1, Five = 1 |
| 0x80000000 + 0x80000000 | Result = 0x00000000, N= 0, Z=1, C = i, V = i |
| 0x00001234 − 0x00001000 | Outcome = 0x00000234, North= 0, Z=0, C = 1, V = 0 |
| 0x00000004 − 0x00000005 | Result = 0xFFFFFFFF, North= 1, Z=0, C = 0, Five = 0 |
| 0xFFFFFFFF − 0xFFFFFFFC | Result = 0x00000003, N= 0, Z=0, C = 1, V = 0 |
| 0x80000005 − 0x80000004 | Issue = 0x00000001, N= 0, Z=0, C = 1, 5 = 0 |
| 0x70000000 − 0xF0000000 | Result = 0x80000000, Due north= ane, Z=0, C = 0, 5 = 1 |
| 0xA0000000 − 0xA0000000 | Effect = 0x00000000, N= 0, Z=ane, C = 1, V = 0 |
In the ARMv7-Yard and ARMv7E-Chiliad architecture, most of the sixteen-bit instructions affect these four ALU flags. In nigh of the 32-bit instructions one of the bits in the teaching encoding defines if the APSR flags should be updated or non. Annotation that some of these instructions practice not update the V flag or the C flag. For case, the MULS (multiply) instruction only changes the North flag and the Z flag.
In addition to provisional branch or provisional execution lawmaking, the Carry scrap of APSR tin also be used to extend add and decrease operations to over 32 bits. For instance, when adding 2 64-bit integers together, nosotros can utilise the carry bit from the lower 32-bit add together operation as an extra input for the upper 32-fleck add together operation:
// Calculating Z = X + Y, where X, Y and Z are all 64-bit
Z[31:0] = X[31:0] + Y[31:0]; // Calculate lower discussion add-on, // carry flag become updated
Z[63:32] = X[63:32] + Y[63:32] + Carry; // Calculate upper // word addition
The North-Z-C-V flags are available in all ARM® processors including the Cortex-M0 processor.
4.3.2 Q status flag
The Q is used to indicate an occurrence of saturation during saturation arithmetic operations or saturation adjustment operations. Information technology is available in ARMv7-Thousand (eastward.g., Cortex®-M3 and Cortex-M4 processors), but not ARMv6-M (e.m., Cortex-M0 processor). After this bit is set, it remains fix until a software write to the APSR clears the Q fleck. Saturation arithmetic/adjustment operations practice non clear this bit. As a upshot, you tin can use this bit to determine if saturation occurred at the terminate of a sequence of Saturation arithmetics/adjustment operations, without the need to check the saturation status during each step.
Saturation arithmetic is useful for digital signal processing. In some cases, the destination register used to hold a calculation consequence might not have sufficient bit width and as a result, overflow or underflow occurs. If normal data arithmetics instructions are used, the MSB of the result would be lost and can cause a serious distortion in the output. Instead of just cutting off the MSB, saturation arithmetic forces the outcome to the maximum value (in example of overflow) or minimum value (in case of underflow) to reduce the bear on of signal baloney (figure 4.xvi).
FIGURE 4.16. Signed saturation and unsigned saturation
The actual maximum and minimum values that trigger the saturation depend on the instructions beingness used. In nearly cases, the instructions for saturation arithmetic are mnemonic starting with "Q," for example "QADD16". If saturation occurred, the Q flake is set for the following instructions - QADD, QDADD, QSUB, QDSUB, SSAT, SSAT16, USAT, USAT16; otherwise, the value of the Q flake is unchanged.
The Cortex-M3 processor provides a couple of saturation aligning instructions, and the Cortex-M4 provides a full gear up of saturation arithmetic instructions, besides as those saturation adjustment instructions available in the Cotex-M3 processor.
four.3.3 GE $.25
The "Greater-Equal" (GE) is a four-bit wide field in the APSR in the Cortex®-M4, and is not available in the Cortex-M3 processor. It is updated past a number of SIMD instructions where, in most cases, each bit represents positive or overflow of SIMD operations for each byte (Table iv.eight). For SIMD instructions with 16-bit data, bit 0 and bit i are controlled by the issue of lower one-half-discussion, and bit 2 and bit 3 are controlled by the outcome of upper half-give-and-take.
Table four.8. GE Flags Results
| SIMD Operation | Results |
|---|---|
| SADD16, SSUB16, USUB16, SASX, SSAX | If lower half-word result >= 0 and then GE[ane:0] = 2'b11 else GE[1:0] = 2'b00 If upper half-word result >= 0 then GE[3:2] = 2'b11 else GE[3:2] = 2'b00 |
| UADD16 | If lower half-word issue >= 0x10000 then GE[ane:0] = ii'b11 else GE[ane:0] = 2'b00 If upper half-discussion outcome >= 0x10000 then GE[three:2] = 2'b11 else GE[three:2] = 2'b00 |
| SADD8, SSUB8, USUB8 | If byte 0 effect >= 0 and then GE[0] = 1'b1 else GE[0] = ane'b0 If byte 1 result >= 0 so GE[1] = 1'b1 else GE[1] = 1'b0 If byte ii effect >= 0 and so GE[2] = one'b1 else GE[two] = 1'b0 If byte three effect >= 0 then GE[3] = ane'b1 else GE[3] = 1'b0 |
| UADD8 | If byte 0 result >= 0x100 then GE[0] = 1'b1 else GE[0] = 1'b0 If byte 1 result >= 0x100 and so GE[one] = one'b1 else GE[i] = 1'b0 If byte two result >= 0x100 then GE[2] = 1'b1 else GE[two] = i'b0 If byte 3 outcome >=0x100 then GE[iii] = 1'b1 else GE[3] = 1'b0 |
| UASX | If lower one-half-word result >= 0 then GE[1:0] = two'b11 else GE[1:0] = 2'b00 If upper one-half-discussion result >= 0x10000 then GE[iii:2] = ii'b11 else GE[iii:2] = 2'b00 |
| USAX | If lower half-word result >= 0x10000 then GE[one:0] = two'b11 else GE[1:0] = ii'b00 If upper half-word result >= 0x0 and then GE[iii:2] = 2'b11 else GE[3:2] = 2'b00 |
The GE flags are used by the SEL didactics(Effigy 4.17), which multiplexes the byte values from two source registers based on each GE scrap. When combining SIMD instructions with the SEL instruction, simple conditional data pick tin be created in SIMD organisation for improve performance.
FIGURE 4.17. SEL operation
Yous tin likewise read back the GE bits by reading APSR into a general purpose register for additional processing. More than details of the SIMD and SEL instructions are given in Chapter 5.
Read full chapter
URL:
https://www.sciencedirect.com/science/commodity/pii/B978012408082900004X
Floating Point Operations
Joseph Yiu , in The Definitive Guide to ARM® CORTEX®-M3 and CORTEX®-M4 Processors (Third Edition), 2014
13.2.5 Floating bespeak condition and control register (FPSCR)
The FPSCR holds the arithmetic issue flags and sticky status flags, every bit well equally fleck fields to command the behavior of the floating point unit of measurement (Effigy xiii.14 and Table 13.four). The N, Z, C, and V flags are updated past floating point comparison operations, as shown in Tabular array 13.5.
Table 13.iv. Bit Fields in FPSCR
| Bit | Clarification |
|---|---|
| North | Negative flag (update past floating point comparison operations) |
| Z | Zero flag (update by floating indicate comparison operations) |
| C | Carry/borrow flag (update by floating point comparing operations) |
| V | Overflow flag (update past floating indicate comparison operations) |
| AHP | Alternate one-half precision command chip: 0 – IEEE one-half-precision format (default) one – Alternative one-half-precision format, see section 13.1.3 |
| DN | Default NaN (Non a Number) mode control bit: 0 – NaN operands propagate through to the output of a floating point operation (default) 1 – Any operation involving ane or more NaN(s) returns the default NaN |
| FZ | Affluent-to-nothing model control bit: 0 – Affluent-to-naught manner disabled (default) (IEEE 754 standard compliant) 1 – Flush-to-zero mode enabled; denormalized values (tiny values with exponent equal 0) are flushed 0 |
| RMode | Rounding Mode Command field; the specified rounding mode is used by almost all floating indicate instructions: 00 – Round to Nearest (RN) mode (default) 01 – Round towards Plus Infinity (RP) mode x – Round towards Minus Infinity (RM) fashion 11 – Round towards Nil (RZ) way |
| IDC | Input Denormal cumulative exception bit; fix to 1 when floating point exception occurred, clear past writing 0 to this flake |
| IXC | Inexact cumulative exception chip; set up to 1 when floating indicate exception occurred, clear by writing 0 to this bit |
| UFC | Underflow cumulative exception chip; gear up to ane when floating point exception occurred, clear past writing 0 to this bit |
| OFC | Overflow cumulative exception bit; ready to i when floating bespeak exception occurred, articulate by writing 0 to this bit |
| DZC | Sectionalization past Zero cumulative exception bit; set to 1 when floating point exception occurred, clear by writing 0 to this fleck |
| IOC | Invalid Performance cumulative exception fleck; set to one when floating signal exception occurred, clear past writing 0 to this scrap |
Figure 13.14. Flake field in FPSCR
Table thirteen.5. Operation of Northward, Z, C, and V Flags in FPSCR
| Comparison Consequence | N | Z | C | V |
|---|---|---|---|---|
| Equal | 0 | 1 | i | 0 |
| Less than | 1 | 0 | 0 | 0 |
| Greater than | 0 | 0 | i | 0 |
| Unordered | 0 | 0 | one | 1 |
You can utilise the results of a floating signal compare for conditional branch/conditional execution by copying the flags to APSR kickoff:
VMRS APSR_nzcv, FPSCR ; Re-create flags from FPSCR to flags in APSR
The fleck fields AHP, DN, and FZ are control register bits for special operation modes. By default, all these bits default to 0 and their beliefs is compliant with IEEE 754 single precision performance. In normal applications there is no need to alter the settings of the floating point operation control. Do not change these $.25 if your application requires IEEE 754 compliance.
The RMode bit field is for controlling the rounding mode for calculation results. The IEEE 754 standard defines several rounding modes, as shown in Table 13.6.
Table xiii.6. Rounding Modes Available on the Cortex®-M4 FPU
| Rounding Style | Descriptions |
|---|---|
| Round to nearest | Rounds to the nearest value. This is the default configuration. IEEE 754 subdivides this mode to: Round to nearest, ties to even: round to the nearest value with an fifty-fifty (zero) LSB. This is the default for binary floating bespeak and recommended default for decimal floating point. Round to nearest, ties away from nada: round to the nearest value above (for +ve numbers) or below (for –ve numbers). This is intended every bit an option for decimal floating point. Since the floating point unit is using binary floating point only, the "Circular to nearest, ties away from zero" mode is not available. |
| Round toward +∞ | Also known as rounding up or ceiling. |
| Round toward −∞ | Likewise known as rounding down or flooring. |
| Round toward 0 | Also known equally truncation. |
The $.25 IDC, IXC, UFC, OFC, DZC, and IOC are viscid condition flags that show any abnormalities (floating point exceptions) during floating point operations. Software can check these flags afterwards the floating signal operations, and can clear them by writing nix to them. Section 13.5 has more information on floating bespeak exceptions.
Read full chapter
URL:
https://world wide web.sciencedirect.com/science/article/pii/B9780124080829000130
Error Exceptions and Error Handling
Joseph Yiu , in The Definitive Guide to ARM® CORTEX®-M3 and CORTEX®-M4 Processors (Third Edition), 2014
12.four.2 Information for MemManage fault
The programmer's model for the MemManage Fault Condition Annals is shown in Tabular array 12.3.
Tabular array 12.3. MemManage Fault Status Register (lowest byte in SCB->CFSR)
| Bits | Proper name | Type | Reset Value | Description |
|---|---|---|---|---|
| 7 | MMARVALID | – | 0 | Indicates the MMFAR is valid |
| 6 | – | – | – (read as 0) | Reserved |
| 5 | MLSPERR | R/Wc | 0 | Floating point lazy stacking error (available on Cortex®-M4 with floating signal unit just) |
| 4 | MSTKERR | R/Wc | 0 | Stacking error |
| iii | MUNSTKERR | R/Wc | 0 | Unstacking mistake |
| two | – | – | – (read as 0) | Reserved |
| 1 | DACCVIOL | R/Wc | 0 | Information access violation |
| 0 | IACCVIOL | R/Wc | 0 | Instruction access violation |
Each fault indication status bit (non including MMARVALID) is set when the fault occurs, and stays loftier until a value of 1 is written to the annals.
If the MMFSR indicates that the mistake is a information access violation (DACCVIOL set to 1) or an didactics access violation (IACCVIOL prepare to 1), the faulting code tin can be located past the stacked plan counter in the stack frame.
If the MMARVALID fleck is set, it is as well possible to make up one's mind the retentiveness location that caused the fault by using the MemManage Fault Accost Register (SCB->MMFAR).
MemManage faults which occur during stacking, unstacking, and lazy stacking (encounter sections viii.3.6 and thirteen.3) are indicated by MSTKERR, MUNSTKERR, and MLSPERR, respectively.
Read full chapter
URL:
https://www.sciencedirect.com/science/article/pii/B9780124080829000129
INTRODUCTION TO THE ARM INSTRUCTION Gear up
ANDREW N. SLOSS , ... CHRIS WRIGHT , in ARM System Developer'south Guide, 2004
3.5 Plan STATUS Register INSTRUCTIONS
The ARM instruction set provides two instructions to straight command a plan status annals ( psr). The MRS instruction transfers the contents of either the cpsr or spsr into a register; in the contrary management, the MSR instruction transfers the contents of a register into the cpsr or spsr. Together these instructions are used to read and write the cpsr and spsr.
In the syntax you tin run into a characterization called fields. This can be whatsoever combination of control (c), extension (ten), status (s), and flags (f). These fields relate to particular byte regions in a psr, equally shown in Effigy 3.9.
Effigy three.9. psr byte fields.
| MRS | copy program condition register to a general-purpose register | Rd = psr |
| MSR | move a general-purpose register to a plan condition annals | psr[field] = Rm |
| MSR | move an immediate value to a program status annals | psr[field] = immediate |
The c field controls the interrupt masks, Thumb state, and processor mode. Example iii.26 shows how to enable IRQ interrupts past clearing the I mask. This operation involves using both the MRS and MSR instructions to read from and so write to the cpsr.
EXAMPLE 3.26
The MSR first copies the cpsr into register r1. The BIC teaching clears chip 7 of r1. Register r1 is then copied back into the cpsr, which enables IRQ interrupts. You can run across from this example that this lawmaking preserves all the other settings in the cpsr and just modifies the I flake in the control field.
This instance is in SVC mode. In user style yous can read all cpsr bits, but y'all can but update the condition flag field f.
3.5.ane COPROCESSOR INSTRUCTIONS
Coprocessor instructions are used to extend the educational activity prepare. A coprocessor tin either provide boosted computation capability or be used to command the memory subsystem including caches and retentiveness management. The coprocessor instructions include data processing, register transfer, and memory transfer instructions. Nosotros will provide only a brusque overview since these instructions are coprocessor specific. Note that these instructions are simply used past cores with a coprocessor.
| CDP | coprocessor data processing—perform an operation in a coprocessor |
| MRC MCR | coprocessor annals transfer—move information to/from coprocessor registers |
| LDC STC | coprocessor retentiveness transfer—load and shop blocks of memory to/from a coprocessor |
In the syntax of the coprocessor instructions, the cp field represents the coprocessor number betwixt p0 and p15. The opcode fields describe the operation to have place on the coprocessor. The Cn, Cm, and Cd fields draw registers within the coprocessor. The coprocessor operations and registers depend on the specific coprocessor you are using. Coprocessor 15 (CP15) is reserved for arrangement control purposes, such equally retentivity management, write buffer command, enshroud command, and identification registers.
Instance iii.27
This example shows a CP15 register existence copied into a general-purpose annals.
Hither CP15 register-0 contains the processor identification number. This annals is copied into the full general-purpose register r10.
three.5.2 COPROCESSOR fifteen Pedagogy SYNTAX
CP15 configures the processor cadre and has a set of defended registers to store configuration information, as shown in Case 3.27. A value written into a annals sets a configuration aspect—for example, switching on the cache.
CP15 is called the system control coprocessor. Both MRC and MCR instructions are used to read and write to CP15, where annals Rd is the core destination annals, Cn is the primary register, Cm is the secondary register, and opcode2 is a secondary annals modifier. You may occasionally hear secondary registers called "extended registers."
As an example, here is the instruction to move the contents of CP15 command register c1 into register r1 of the processor core:
We use a shorthand notation for CP15 reference that makes referring to configuration registers easier to follow. The reference note uses the following format:
The first term, CP15, defines it as coprocessor 15. The 2d term, later the separating colon, is the primary annals. The primary register X can have a value between 0 and 15. The third term is the secondary or extended annals. The secondary register Y can have a value between 0 and 15. The concluding term, opcode2, is an pedagogy modifier and can have a value between 0 and vii. Some operations may also use a nonzero value due west of opcode1. We write these as CP15:w:cX:cY:Z.
Read full chapter
URL:
https://www.sciencedirect.com/science/article/pii/B9781558608740500046
Advanced PIC18 Projects
Dogan Ibrahim , in PIC Microcontroller Projects in C (2d Edition), 2014
SSPxSTAT
This is the status register with the lower half dozen bits read only and the upper 2 bits read/write. Figure 7.44 shows the bit definitions of this register. Only $.25 0, vi, and 7 are related to functioning in the SPI manner. Bit seven (SMP) allows the user to select the input information sample time. When SMP = 0, input data are sampled at the center of data output time, and when SMP = 1, the sampling is done at the stop. Chip half-dozen (CKE) allows the user to select the transmit clock edge. When CKE = 0, transmit occurs on transition from the idle to active clock state, and when CKE = ane, transmit occurs on transition from the active to the idle clock state. Bit 0 (BF) is the buffer total status bit. When BF = 1, receive is complete (i.e. SSPxBUF is total), and when BF = 0, receive is non consummate (i.e. SSPxBUF is empty).
Figure 7.44. SSPxSTAT Register Flake Configuration. Just the Bits when Operating in the SPI Master Mode are Described.
Read total chapter
URL:
https://www.sciencedirect.com/science/article/pii/B9780080999241000071
Source: https://www.sciencedirect.com/topics/engineering/status-register
Posted by: adamstogre1996.blogspot.com
0 Response to "Does Bit Clearing The Flags In Status Register Do Anything"
Post a Comment