James Newton says:
Despite the fact that it is not documented here, any operation (add or sub, jmp PC+W, jmp W, mov, etc...) that modifies PC, other than JMP addr, will set bit 8 of the PC to zero. So you can only do relative or computed jumps into the first half page of each page. Just like calls.
SX User’s Manual Rev. 3.1 42 © 2000 Scenix Semiconductor, Inc. All rights reserved. www.scenix.com Chapter 2 Architecture Another  way  to  achieve  the  same  effect  faster  and  with  fewer  instructions  is  to  use  the  “page”
instruction to set the PA2:PA0 bits in the STATUS register: page  $600 ;set page to 600h (PA2:PA0 = 011 binary) jmp $1E0 ;jump to program memory address 7E0h The “page” instruction sets the values of the PA2:PA0 bits without affecting other bits in the STATUS
register.  It  does  this  in  just  one  clock  cycle.  You  specify  a  12-bit  value in  the  instruction  and  the
assembler encodes the three high-order bits of the value into the instruction (and ignores the lower-
order  bits).  When  you  execute  the  instruction,  it  sets  the  PA2:PA0  bits  in  the  STATUS  register
accordingly. Note that is necessary to set the PA2:PA0 bits prior to the “jmp” instruction only if they do not already
contain  the  desired  page  number.  You  can  set  them  just  once  and  then  use  any  number  of  “jmp”
instructions as long as you stay within the same 512-word page in the program memory. A “JMP addr9” instruction takes two clock cycles in the “compatible” clocking mode (SX18/20/28AC
and  SX18/20/28AC75  devices  only)  or  three  clock  cycles  in  the  “turbo”  clocking  mode.  (For
information on clocking modes, see Section 4.2). 2.6.3 Jump Indirect and Jump Relative Instead of using the “JMP addr9” to specify an absolute jump destination, you can cause a jump by
modifying the PC register (file register address 02h), which holds the lower eight bits of the program
counter. For example, to perform an indirect jump, you can move a new value from W to PC, as in the following
example: mov W,$0B ;load W with 8-bit jump address from file reg. mov $02,W ;load PC with new address (lower 8 bits only) To perform an indirect relative jump (a jump of a certain number of memory locations forward or
backward from the next instruction), you can add W to PC or subtract W from PC, as in the following
example: mov W,#$04   ;load W with the immediate value 04h add $02,W ;increase PC by 4 (jump forward 5 instructions) You can use an indirect jump to implement a multiple-branch conditional jump (for example, to jump
to one of four different routines based on a calculation result). If you perform a jump by modifying the PC register, you can only jump to a location within the same
256-word segment in the program memory. This is because you can only modify the lower eight bits
of  the program counter. To  jump  across a  256-word boundary,  use the “PAGE addr12” and “JMP
addr9” instructions. A jump performed by modifying the PC register with a “mov” or “add” instruction takes four clock
cycles in the “compatible” clocking mode (SX18/28/28AC and SX18/20/28AC75 devices only) or
three clock cycles in the “turbo” clocking mode.