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#### 11.6.2 Functions and Variables Specifying the Register Architecture

These `struct gdbarch` functions and variables specify the number and type of registers in the architecture.

— Architecture Function: CORE_ADDR read_pc (struct regcache *regcache)
— Architecture Function: void write_pc (struct regcache *regcache, CORE_ADDR val)

> Read or write the program counter. The default value of both functions is `NULL` (no function available). If the program counter is just an ordinary register, it can be specified in `struct gdbarch` instead (see `pc_regnum` below) and it will be read or written using the standard routines to access registers. This function need only be specified if the program counter is not an ordinary register.

> Any register information can be obtained using the supplied register cache, regcache. See Register Caching.

— Architecture Function: void pseudo_register_read (struct gdbarch *gdbarch, struct regcache *regcache, int regnum, const gdb_byte *buf)
— Architecture Function: void pseudo_register_write (struct gdbarch *gdbarch, struct regcache *regcache, int regnum, const gdb_byte *buf)

> These functions should be defined if there are any pseudo-registers. The default value is `NULL`. regnum is the number of the register to read or write (which will be a cooked register number) and buf is the buffer where the value read will be placed, or from which the value to be written will be taken. The value in the buffer may be converted to or from a signed or unsigned integral value using one of the utility functions (see Using Different Register and Memory Data Representations).

> The access should be for the specified architecture, gdbarch. Any register information can be obtained using the supplied register cache, regcache. See Register Caching.

— Architecture Variable: int sp_regnum

> This specifies the register holding the stack pointer, which may be a raw or pseudo-register. It defaults to -1 (not defined), but it is an error for it not to be defined.

> The value of the stack pointer register can be accessed withing gdb as the variable `$sp`.

— Architecture Variable: int pc_regnum

> This specifies the register holding the program counter, which may be a raw or pseudo-register. It defaults to -1 (not defined). If `pc_regnum` is not defined, then the functions `read_pc` and `write_pc` (see above) must be defined.

> The value of the program counter (whether defined as a register, or through `read_pc` and `write_pc`) can be accessed withing gdb as the variable `$pc`.

— Architecture Variable: int ps_regnum

> This specifies the register holding the processor status (often called the status register), which may be a raw or pseudo-register. It defaults to -1 (not defined).

> If defined, the value of this register can be accessed withing gdb as the variable `$ps`.

— Architecture Variable: int fp0_regnum

> This specifies the first floating point register. It defaults to 0. `fp0_regnum` is not needed unless the target offers support for floating point.