Among 19 programmer-visible registers there are two main groups of 8 each and 3 special registers scattered around the CPU:

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1) General Purpose Registers: 8 16-bit registers that are accessible to main ALU.

These registers are named r0, r1, r2, r3, r4, r5, r6 and r7.

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2) Memory Pointer Registers: 8 16-bit registers combined in 4 pairs to hold 32-bit values. The registers are also accessible individually for data transfer. 

Memory Pointer Pair (32-bit) names, they are used in Indirect Memory Access instructions, Load Immediate into Pointer instruction and Address Arithmetic instructions:

PC -- Program Counter, updated each time new instruction is fetched

SP -- Stack Pointer, provides address for storing current PC value when executing JSR instruction

FP -- Frame Pointer, can be used as additional pointer to manage program stack

BP -- Base Pointer, can be used as additional pointer, for example, to some data structure in memory, like object or array.

Individual Memory Pointer register (16-bit) names:

mp0, or PCH -- high word of PC pair

mp1, or PCL -- low word of PC pair

mp2, or SPH -- high word of SP pair

mp3, or SPL -- low word of SP pair

mp4, or FPH -- high word of FP pair

mp5, or FPL -- low word of FP pair

mp6, or BPH -- high word of BP pair

mp7, or BPL -- low word of BP pair

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3) Status Register

SR -- this register is only 4-bits, it contains flags indicating ALU operation output characteristics, such as if there is a carry generated, or oveflow, or the resulting value is negative or zero.

It can have its value moved into or from one of GPR or MPR  with special MOV operation.

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4) Memory Data Buffer 

MDB -- 16-bit register -- written to automatically whenever F2 state is active by the value of second word of 2-word instruction.

It can also be written to or read from to GPR or MPR  manually with special MOV operation.

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5) Interrupt Vector Base

IVB -- 16-bit register, used to provide the base address for an array of Interrupt Service Routines, is a high word of 32-bit address.

Can also be written to or read from to GPR or MPR with special MOV operation. The write is used for initial setup of system.