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27
μ
PD75518(A)
3. INTERNAL CPU FUNCTIONS
3.1 PROGRAM COUNTER (PC): 15 BITS
The program counter is a 15-bit binary counter for holding program memory address information.
Fig. 3-1 Program Counter Format
Note that the reset start address must be set within a space of 16K bytes (0000H to 3FFFH). This is because
a RESET input sets the low-order six bits of program memory address 0000H in PC13 to PC8, and the contents
of address 0001H in PC7 to PC0, and 0 in PC14 for initialization.
3.2 PROGRAM MEMORY (ROM): 32640 WORDS
×
8 BITS
The program memory is a mask-programmable ROM with a configuration of 32640 words
×
8 bits for storing
programs, table data, and so forth.
Program memory is addressed by the program counter. Table data can be referenced using the table
reference instruction (MOVT).
Fig. 3-2 shows the allowable branch address ranges for the branch instructions and subroutine call
instructions. The whole-space branch instruction (BRA !addr1) and the whole-space call instruction (CALLA
!addr1) allow a direct branch throughout the whole space 0000H to 5F7FH. The relative branch instruction
(BR $addr) allows a branch to addresses (PC - 15 to PC - 1 and PC + 2 to PC + 16) regardless of block boundaries.
The program memory is located at addresses 0000H to 5F7FH containing the following specially assigned
addresses. (All areas excluding 0000H and 0001H can be used as normal program memory.)
0000H to 0001H
Vector table for holding the RBE and MBE setting values and program start address at the time of a RESET
input. A reset start can be performed at an arbitrary address within a 16K-byte space (0000H to 3FFFH).
0002H to 000DH
Vector table for holding the RBE and MBE setting values and program start address at the time of each
vectored interrupt occurrence. Interrupt processing can be started at an arbitrary address within a 16K-
byte space (0000H to 3FFFH).
0020H to 007FH
Table area referenced by the GETI instruction
Note
Note
The GETI instruction can represent an arbitrary 2-byte or 3-byte instruction or two 1-byte instructions
in 1 byte, thus reducing the number of program steps. (See
Section 8.1
.)
PC12
PC11
PC10
PC9
PC8
PC7
PC6
PC5
PC4
PC3
PC2
PC1
PC0
PC
PC13
PC14