23b1ce7e_SP0256_Instruction_Set__Purpose
[Register] COMMAND [Size] 8 bits [Purpose] 
This holds address of the most recent command from the host CPU. Addresses are loaded into this register via external pins and the ALD control line. When the microsequencer is halted (or is about to halt), it watches for an address in this register. When a new command address is available, it copies these bits to bits 1 through 8 of the program counter. Bits 0, 9 through 11, and 13 through 15 are forced to zero. Bit 12 is forced to 1 so that code executes out of page $1.
SP0256 Instruction Set
Purpose