Hugh O'Byrne
Continued from the first half of this article

In this type of situation, setting the page after the GOTO destination is more common, as in the latter example. If there are several GOTOs to the same label, setting the page once after the label is usually better than doing so several times before the GOTOs. This strategy makes it easier to modify the code later; if you add another GOTO, you don't have to remember to set Page beforehand.

Shorthand

If you were to follow this method rigorously, you'd need a comment for nearly every instruction and label in the program, which would make the wordiness of the comments in the previous examples tiresome. For shorthand, I use four characters to show the possible paging values. I use a - symbol for a page that is definitely not selected, and a # symbol for a page that may be, or definitely is, selected. Thus, when the current page is unknown, the comment would look like: ;####. When you know the active page is page 1, the comment would be ;-#-- (taking the first character to represent page 0). When the active page may be either 1 or 2, the comment would be ;-##-. In addition, I put comments on the same line as the previous instruction (or label). Thus, using the second fix described above, the code fragments (put together) look like this:

#define DataReg rD
#define ChecksumReg_P0 r5
#define CounterReg_P2 r0
#define MaskReg_P2 r3
...

Label1:			;####
	A <-X imm	2	;####
	Page <-A		;--#-
	A <-A+X imm	1	;--#-
	X<-A reg	CounterReg_P2	;--#-
	GOTO imm	WhatNext
...
WhatNext:			;#-#-
	A <-X imm	2	;#-#-
	Page <-X		;--#-
	A <-X reg	MaskReg_P2	;--#-
...
Label2:			;####
	A <-X imm	0	;####
	Page<-A		;#---
	A <-X reg	ChecksumReg_P0	;#---
	A < -A+X	reg DataReg	;#---
	X<-A reg	ChecksumReg_P0	;#---
	GOTO imm	WhatNext

Note that the rule for GOTOs and GOTO destinations is not as clear as for the other instructions: from one place, page 2 may be active before a GOTO instruction and from another place, page 0 may be active before a GOTO instruction. If they both jump to the same place in the program, that place may have either page 2 or page 0 active. Symbolically, where the comment before the GOTO has a #, so must the comment after the label. Where the comment before a GOTO has a -, the comment after the label may have either symbol.

Summary of the rules to date

This is a good point to take stock of what we've learned. The logical rules we have gathered are as follows:

• L1: variables must be identified and tracked not by ambiguous logical registers, but by unambiguous physical registers
• L2: wherever a paged variable is referenced, the page associated with the variable must be activeýand equally important, all other pages must be inactive
• L3: an instruction that modifies Page will change the active page
• L4: any page that may be active before an instruction that does not modify Page and is not a flow-control instruction may be active after that instruction
• L5: any page that may be active before a flow-control instruction may be active at the destination(s)
• L6: any page that may be active before encountering a label may be active after the label