G.5.3 ARM Features

G.5.3.1 Core register set for non-M-profile

The ‘org.gnu.gdb.arm.core’ feature is required for non-M-profile ARM targets. It must contain the following registers:

• - ‘r0’ through ‘r12’. The general purpose registers. They are 32 bits in size and have a type of ‘uint32’.
• - ‘sp’, the stack pointer register, also known as ‘r13’. It is 32 bits in size and has a type of ‘data_ptr’.
• - ‘lr’, the link register. It is 32 bits in size.
• - ‘pc’, the program counter register. It is 32 bit in size and of type ‘code_ptr’.
• - ‘cpsr’, the current program status register containing all the status bits. It is 32 bits in size. Historically this register was hardwired to number 25, but debugging stubs that report XML do not need to use this number anymore.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.2 Core register set for M-profile

For M-profile targets (e.g. Cortex-M3), the ‘org.gnu.gdb.arm.core’ feature is replaced by ‘org.gnu.gdb.arm.m-profile’, and it is a required feature. It must contain the following registers:

• - ‘r0’ through ‘r12’, the general purpose registers. They have a size of 32 bits and a type of ‘uint32’.
• - ‘sp’, the stack pointer register, also known as ‘r13’. It has a size of 32 bits and a type of ‘data_ptr’.
• - ‘lr’, the link register. It has a size of 32 bits.
• - ‘pc’, the program counter register. It has a size of 32 bits and a type of ‘code_ptr’.
• - ‘xpsr’, the program status register containing all the status bits. It has a size of 32 bits. Historically this register was hardwired to number 25, but debugging stubs that report XML do not need to use this number anymore.

Upon seeing this feature, GDB will acknowledge that it is dealing with an M-profile target. This means GDB will use hooks and configurations that are meaningful to M-profiles.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.3 FPA registers feature (obsolete)

The ‘org.gnu.gdb.arm.fpa’ feature is obsolete and should not be advertised by debugging stubs anymore. It used to advertise registers for the old FPA architecture that has long been discontinued in toolchains.

It is kept in GDB for backward compatibility purposes so older debugging stubs that don’t support XML target descriptions still work correctly. One such example is the KGDB debugging stub from Linux or BSD kernels.

The description below is for historical purposes only. This feature used to contain the following registers:

• - ‘f0’ through ‘f8’. The floating point registers. They are 96 bits in size and of type ‘arm_fpa_ext’. ‘f0’ is pinned to register number 16.
• - ‘fps’, the status register. It has a size of 32 bits.

G.5.3.4 M-profile Vector Extension (MVE)

Also known as Helium, the M-profile Vector Extension is advertised via the optional ‘org.gnu.gdb.arm.m-profile-mve’ feature.

It must contain the following:

• - ‘vpr’, the vector predication status and control register. It is 32 bits in size and has a custom flags type. The flags type is laid out in a way that exposes the ‘P0’ field from bits 0 to 15, the ‘MASK01’ field from bits 16 to 19 and the ‘MASK23’ field from bits 20 to 23.

  Bits 24 through 31 are reserved.

When this feature is available, GDB will synthesize the ‘p0’ pseudo-register from ‘vpr’ contents.

This feature must only be advertised if the target is M-profile. Advertising this feature for targets that are not M-profile may cause GDB to assume the target is M-profile when it isn’t.

If the ‘org.gnu.gdb.arm.vfp’ feature is available alongside the ‘org.gnu.gdb.arm.m-profile-mve’ feature, GDB will synthesize the ‘q’ pseudo-registers from ‘d’ register contents.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.5 XScale iwMMXt feature

The XScale ‘org.gnu.gdb.xscale.iwmmxt’ feature is optional. If present, it must contain the following:

• - ‘wR0’ through ‘wR15’, registers with size 64 bits and a custom type ‘iwmmxt_vec64i’. ‘iwmmxt_vec64i’ is a union of four other types: ‘uint64’, a 2-element vector of ‘uint32’, a 4-element vector of ‘uint16’ and a 8-element vector of ‘uint8’.
• - ‘wCGR0’ through ‘wCGR3’, registers with size 32 bits and type ‘int’.

The following registers are optional:

• - ‘wCID’, register with size of 32 bits and type ‘int’.
• - ‘wCon’, register with size 32 bits and type ‘int’.
• - ‘wCSSF’, register with size 32 bits and type ‘int’.
• - ‘wCASF’, register with size 32 bit and type ‘int’.

This feature should only be reported if the target is XScale.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.6 Vector Floating-Point (VFP) feature

The ‘org.gnu.gdb.arm.vfp’ feature is optional. If present, it should contain one of two possible sets of values depending on whether VFP version 2 or VFP version 3 is in use.

For VFP v2:

• - ‘d0’ through ‘d15’. The double-precision registers. They are 64 bits in size and have type ‘ieee_double’.
• - ‘fpscr’, the floating-point status and control register. It has a size of 32 bits and a type of ‘int’.

For VFP v3:

• - ‘d0’ through ‘d31’. The double-precision registers. They are 64 bits in size and have type ‘ieee_double’.
• - ‘fpscr’, the floating-point status and control register. It has a size of 32 bits and a type of ‘int’.

If this feature is available, GDB will synthesize the single-precision floating-point registers from halves of the double-precision registers as pseudo-registers.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.7 NEON architecture feature

The ‘org.gnu.gdb.arm.neon’ feature is optional. It does not need to contain registers; it instructs GDB to display the VFP double-precision registers as vectors and to synthesize the quad-precision registers from pairs of double-precision registers. If this feature is present, ‘org.gnu.gdb.arm.vfp’ must also be present and include 32 double-precision registers.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.8 M-profile Pointer Authentication and Branch Target Identification feature

The ‘org.gnu.gdb.arm.m-profile-pacbti’ feature is optional, and acknowledges support for the ARMv8.1-m PACBTI extensions.

This feature doesn’t contain any required registers, and it only serves as a hint to GDB that the debugging stub supports the ARMv8.1-m PACBTI extensions.

When GDB sees this feature, it will track return address signing states and will decorate backtraces using the [PAC] marker, similar to AArch64’s PAC extension. See AArch64 PAC.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.9 M-profile system registers feature

The ‘org.gnu.gdb.arm.m-system’ optional feature was introduced as a way to inform GDB about additional system registers.

At the moment this feature must contain the following:

• - ‘msp’, the main stack pointer register. It is 32 bits in size with type ‘data_ptr’.
• - ‘psp’, the process stack pointer register. It is 32 bits in size with type ‘data_ptr’.

This feature must only be advertised for M-profile targets. When GDB sees this feature, it will attempt to track the values of ‘msp’ and ‘psp’ across frames.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.10 M-profile Security Extensions feature

The ‘org.gnu.gdb.arm.secext’ optional feature was introduced so GDB could better support the switching of stack pointers and secure states in the Security Extensions.

At the moment this feature must contain the following:

• - ‘msp_ns’, the main stack pointer register (non-secure state). It is 32 bits in size with type ‘data_ptr’.
• - ‘psp_ns’, the process stack pointer register (non-secure state). It is 32 bits in size with type ‘data_ptr’.
• - ‘msp_s’, the main stack pointer register (secure state). It is 32 bits in size with type ‘data_ptr’.
• - ‘psp_s’, the process stack pointer register (secure state). It is 32 bits in size with type ‘data_ptr’.

When GDB sees this feature, it will attempt to track the values of all 4 stack pointers across secure state transitions, potentially improving unwinding when applications switch between security states.

Extra registers are allowed in this feature, but they will not affect GDB.

G.5.3.11 TLS registers feature

The optional ‘org.gnu.gdb.arm.tls’ feature contains TLS registers.

Currently it contains the following:

• - ‘tpidruro’, the user read-only thread id register. It is 32 bits in size and has type ‘data_ptr’.

At the moment GDB looks for this feature, but doesn’t do anything with it other than displaying it.

Extra registers are allowed in this feature, but they will not affect GDB.