© 2000 Scenix Semiconductor, Inc. All rights reserved. 43 SX User’s Manual Rev. 3.1 www.scenix.com Chapter 2 Architecture 2.6.4 Call The “CALL addr8” instruction calls a subroutine. It works just like a “JMP addr9” instruction, with
the following differences: • The “call” instruction saves the full program counter value, incremented by one, on the program
stack. This allows the program to later return from the subroutine and continue execution with
the instruction immediately following the call. • The “call” instruction only specifies the lower eight bits (rather than the lower nine bits) of the
jump address. The ninth bit (bit 8) of the jump address is always 0. Therefore, the subroutine must
start in the bottom half of a 512-word page in the program memory (000h to 0FFh, 200h to 2FFh,
etc.). Figures 2-3 and 2-4 show how the program counter is loaded for a “jmp” instruction and for “call” instruction, respectively. In either case, the PA2:PA0 bits must contain the desired 512-word page of
the  program  memory  before  the  “jmp”  or  “call”  instruction  is  executed.  These  bits  can  be  easily
changed with the “page” instruction.   Figure 2-3 Program Counter Loading for Jump Instruction Figure 2-4 Program Counter Loading for Call Instruction PA2 PA1 PA0 TO PD Z DC C STATUS REGISTER 9-BIT VALUE IN
JMP INSTRUCTION PROGRAM COUNTER PC (7:0)           BITS 11:8 OF
PROGRAM COUNTER PA2 PA1 PA0 TO PD Z DC C STATUS REGISTER 8-BIT VALUE IN
CALL INSTRUCTION PROGRAM COUNTER PC (7:0)           BITS 11:8 OF
PROGRAM COUNTER ZERO