JMPRET/PUSHZC/POPZC instruction notes.

The JMPRET instruction in Prop2 has some slight differences to Prop1.
This instruction now supports the WZ and WC effects.
When a JMPRET (CALL) is assembled with WZ/WC the destination "RET"
address which normally has a JMP PC+1 instruction also has
the current Z and C states stored in the d-field. 

Instruction encodedd as:

000111_0001_1111_000000ZC_sssssssss	'ret          ''sssssssss = pc+1
000111_1101_1111_000000ZC_sssssssss	'ret wz,wc  

To restore the flag states use WZ/WC in the "RET" instruction.

Code example:

		call	#app wz,wc  'z and c saved in app_ret d-field

	
app		'do something here
		.
		.
app_ret		ret wz,wc	'z and c restored

An alternative method of saving flags would be to use
the PUSHZC/POPZC instructions.

The PUSHZC/POPZC instructions rotate ZC states thru the
 source address as follows.

PUSHZC rotates left x 2 thru source address.
Z/C flags are only updated if WZ/WC effects are nominated.

+-----------> Z flag  ----+
|                         |
|  +------->  C flag  ------+
|  |                      | |
WZ WC                     | |
                          | | 
^  ^                      | |
|  |                      V V

31 30.................... 1 0
   <-----------------------
       rotated left x2


POPZC rotates right x 2 thru source address
Z/C flags are only updated if WZ/WC effects are nominated.

+------------ Z flag  <---+
|                         |
|  +--------  C flag  <-----+
|  |                      | |
|  |                     WZ WC
|  |                      ^ ^ 
|  |                      | |
V  V                      | |

31 30.................... 1 0 
   ----------------------->
      rotated right x2

This would allow a 16 level flag stack to be implemented.