EM78P447S
OTP ROM
3. R2 (Program Counter) & Stack
• Depending on the device type, R2 and hardware stack are 10-bit wide. The structure is depicted in
Fig.3.
• Generating 1024×13 bits on-chip OTP ROM addresses to the relative programming instruction codes.
One program page is 1024 words long.
• R2 is set as all "0"s when under RESET condition.
• "JMP" instruction allows direct loading of the lower 10 program counter bits. Thus, "JMP" allows PC
to go to any location within a page.
• "CALL" instruction loads the lower 10 bits of the PC, and then PC+1 is pushed into the stack. Thus,
the subroutine entry address can be located anywhere within a page.
• "RET" ("RETL k", "RETI") instruction loads the program counter with the contents of the top-level
stack.
• "ADD R2,A" allows the contents of ‘A’ to be added to the current PC, and the ninth and tenth bits of
the PC are cleared.
• "MOV R2,A" allows to load an address from the "A" register to the lower 8 bits of the PC, and the ninth
and tenth bits of the PC are cleared.
• Any instruction that writes to R2 (e.g. "ADD R2,A", "MOV R2,A", "BC R2,6",⋅⋅⋅⋅⋅) will cause the ninth
and tenth bits (A8~A9) of the PC to be cleared. Thus, the computed jump is limited to the first 256
locations of a page.
• All instruction are single instruction cycle (fclk/2 or fclk/4) except for the instruction that would change
the contents of R2. Such instruction will need one more instruction cycle.
R3
000H
001H
002H
User Memory
Space
A11 A10 A9 A8
A7
CALL
RET
RETL
RETI
~
A0
Hardware Vector
Software Vector
00 PAGE0 0000~03FF
01 PAGE1 0400~07FF
10 PAGE2 0800~0BFF
11 PAGE3 0C00~0FFF
Stack Level 1
Stack Level 2
Stack Level 3
Stack Level 4
Stack Level 5
On-chip Program
Memory
Reset Vector
FFFH
Fig. 3 Program Counter Organization
This specification is subject to change without prior notice.
9
06.25.2003 (V1.1)
EMC [ ELAN MICROELECTRONICS CORP ]
