Rabbit 2000 Reference Manual

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66 Rabbit 2000/3000 MicroprocessorDescriptionThese instructions are used to access 20-bit addresses. In all cases, the four most significant bits of the 20-bitaddress (bits 19 through 16) are defined as the four least significant bits of the Accumulator (bits 3 though 0).The LDP instructions bypass the MMU’s address translation unit for direct access to the 20-bit memoryaddress space. These instructions are implemented for the Rabbit and are not available for the Z180.• LDP HL,(mn): Loads the register L with the data whose 16 least significant bits of its 20-bitaddress are the 16-bit constant mn, and then loads the register H with the data in the following20-bit address.• LDP IX,(mn): Loads the low order byte of index register IX with the data whose 16 leastsignificant bits of its 20-bit address are the 16-bit constant mn, and then loads the high order byteof IX with the data in the following 20-bit address.• LDP IY,(mn): Loads the low order byte of index register IY with the data whose 16 leastsignificant bits of its 20-bit address are the 16-bit constant mn, and then loads the high order byteof IY with the data in the following 20-bit address.Note that the LDP instructions wrap around on a 64K page boundary. Since the LDP instruction operates ontwo-byte values, the second byte will wrap around and be written at the start of the page if you try to read orwrite across a page boundary. Thus, if you fetch or store at address 0xn,0xFFFF, you will get the byteslocated at 0xn, 0xFFFF and 0xn,0x0000 instead of 0xn,0xFFFF and 0x(n+1)0x0000 as you might expect.Therefore, do not use LDP at any physical address ending in 0xFFFF.LDP HL,(mn)LDP IX,(mn)LDP IY,(mn)Opcode Instruction Clocks OperationED 6D n m LDP HL,(mn) 13* L = (mn); H = (mn + 1).(Addr[19:16] = A[3:0])DD 6D n m LDP IX,(mn) 13* IX (low) = (mn); IX (high) = (mn + 1).(Addr[19:16] = A[3:0])FD 6D n m LDP IY,(mn) 13* IY (low) = (mn); IY (high) = (mn + 1).(Addr[19:16] = A[3:0])*Clocking: 13 (2,2,2,2,1,2,2)Flags ALTD I/OS Z L/V C F R SP S D- - - -