Adding Registers

Adding registers to PMON is very simple. You might want to do this in order to make PMON able to display and modify the registers of peripheral devices that you have on your own board. For example, if you have a real-time clock chip that contains 4 registers. You can add these registers to PMON so that you can display and set them from the PMON command line.

The register information is entered into a table that is located in the appropriate file in the lib directory. For example, if your processor uses the 4001 core, you should add the registers to the array c4001_reglist located in the file lib/c4001.c. A fragment of this table is shown below..

        {mXc0,mips_cause_def,"C0_CAUSE","CAUSE",13,(F_CP0|F_MIPS)},
        {mXmem,0,"M_TIC0","TIC0",M_TMR4001+O_TIC0},
        {mXmem,Tmr4003Stat,"M_TSTAT","TSTAT",M_TMR4001+O_TSTAT},

Reading from left to right, the field definitions are as follows...

The first field contains the address of a function that can be used to set and get the current value of the register. The prototype for a register access function is...
```
	unsigned long mXmem(int mode, int reg, unsigned long value)
```
where:
- mode is either zero for read, or 1 for write.
- reg is the value from the 5th field of the reglist[] structure.
- value (for write operations) contains the value to be written.
- For reads, the return value is the value read from the register.
The first example shows the definition for a register that is accessed via coprocessor 0. The remaining examples are for memory-mapped registers.
The second field contains either zero, or the address of an array of structures that describe the bit fields within the register. For example,
```
char *excodes[] = {
	"Int","MOD","TLBL","TLBS","AdEL","AdES","IBE","DBE",
	"Sys","Bp","RI","CpU","Ovf","Resv","Resv","Resv",0};


	RegSpec mips_cause_def[] = {
	{1,31,"BD",2,0,1},
	{2,28,"CE",10,0,1},
	{6,10,"IP",2,0,1},
	{2,8,"SW",2,0,0},
	{4,2,"EXCODE",0,excodes,1},
	{0}};
```
Where:
- The first field of the structure definition contains the width of the register field in bits. For example, in the example above, the BD field is one bit wide, while the IP field is 6 bits wide.
- The second field of the structure definition contains the least significant bit number of the field. For example, in the example above, the BD field is in bit 31, while the IP field starts at bit 10.
- The third field of the structure definition contains the name of the register field.
- The fourth field of the structure definition contains the numerical base that PMON should use when displaying the value of the register field. Possible values are 2, 8, 10, and 16. If this value is zero, it indicates that the next field if the structure contains a pointer to an array.
- The fifth field of the structure definition contains either zero, or a pointer to an array that specifies the strings that should be displayed for each possible value of the register field. In this example the values of the EXCODE field are specified by the array excodes[].
- The sixth field of the structure definition contains a flag that is used to indicate that this register field is read-only. In the example above, only the SW field is not read-only.
In the first example, the CAUSE and TSTAT registers use this feature to specify the bit fields in the register.
The third field contains the long name of the register.
The fourth field contains the short (or alternate) name of the register.
The fifth field contains additional information that is passed to the register access function.
The sixth field (if present) contains flags that specify additional information about the entry. For example,

F_RO The register is read-only.
R_WO The register is write-only.

Once the table has been modified, you need to rebuild PMON and make new ROMs.

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F_RO	The register is read-only.
R_WO	The register is write-only.