EM78P156N
OTP ROM
3. R2 (Program Counter) & Stack
• Depending on the device type, R2 and hardware stack are 10-bits 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.
000H
008H
Reset Vector
Interrupt Vector
PC (A9 ~ A0)
On-chip Program
Memory
Stack Level 1
Stack Level 2
Stack Level 3
Stack Level 4
Stack Level 5
3FFH
Fig. 3 Program Counter Organization
This specification is subject to change without prior notice.
9
07.29.2004 (V1.2)
ELAN [ ELAN MICROELECTRONICS CORP ]
