VAT Searches

z80 » Variables

• The VAT
• Searching the VAT
• Using _FindAlphaUp/Dn
• Shells

Want to make your own shell? Then keep reading!

Before you learn to search the Variable Allocation Table (VAT or Symbol Lookup Table) we need to investigate where it is and how it's organized.

The VAT

_RAM_Page_7	=$47f3
	call _RAM_Page_7	;swap in ram page 7
				;destroys none

Backwards, that's how it's stored. If you had a variable named 'Stupid', it would be stored as 'diputS' in memory.

Since the VAT is backwards and on RAM page 7, it starts at address $bfff. To swap in RAM page 7 and go to the start of the VAT...

_RAM_Page_7	=$47f3
start_of_vat		=$bfff
	call _RAM_Page_7	;swap in ram page 7
	ld hl,start_of_vat	;hl=beginning of first
				; variable name in
				; vat

There's a good reason behind making it backwards. Think of the variables' memory in the RAM as being one long block. We could store all the names in one area, and all the data in another but we might be wasting space or corrupting memory if the data runs over the name area. By having both areas (name and data) start at opposite ends and work towards the center, you will maximize the allotted memory so that each area only takes up as much space as needed. When you run out of memory for storing names, that means you're out of space to store the data. Want a visual example?

* That's 'Names' spelled backwards since the variable names are backwards in memory.

It's not just the names that are stored backwards, it's the entire variable entry (all the info associated with that variable). Let's say you were to swap in RAM page 7, at $bfff would be the first byte of the first entry in the VAT. Here's an example of what the first variable on page 7 would look like on a freshly reset calculator using my MemSee.asm program.

Address	Byte	ASCII
$bff5	74	t
$bff6	61	a
$bff7	74	t
$bff8	53	S
$bff9	78	x
$bffa	05	
$bffb	00	
$bffc	01	
$bffd	40	@
$bffe	00	
$bfff	04	

After that all, the next variable follows in that same order. Download MemSee.asm to see this first hand if you're still unsure.

Searching the VAT

There's several different ways to go about sifting through the VAT. One way, and probably the most used, is by searching for a certain variable type, most often a string ($0c) or program ($12). Another way is to search alphabetically. We're going to focus on searching by the variable type, and our examples will search for programs to execute.

Let's begin this section by explaining my simple routine to search the VAT: VATSrch.inc. It's a beginner's example of how to search the VAT.

↓_RAM_Page_7		=$47f3
↓_PTEMP_END		=$d29a
↓vatsrch:
↓	call _RAM_Page_7
↓	ld bc,(_PTEMP_END+1)
↓		push hl
↓	or a
↓	sbc hl,bc
↓		pop bc
↓	ret c
↓		push bc
↓	ld b,h
↓	ld c,l
↓		pop hl
↓	cpdr
↓	ld d,a
↓	ld a,b
↓	or c
↓	ld a,d
↓	jr nz,good_found_one
↓	scf
↓	ret
↓good_found_one:
↓	dec hl
↓	dec hl
↓	dec hl
↓	dec hl
↓	ret

	ld a,7
	out (5),a

Check out VATSrch.inc's VATSrch and Copy_to_OP1 in action!

	ld a,$12		;$12 - program
				;$0c - string
	ld hl,$bfff		;absolute start of vat
				;the routine automatically
				; swaps in ram page 7 (the
				; vatiable allocation table)
				; so this is a valid address
	call vatsrch		;search starting at hl for
				; program
	ret c			;carry set tells us there was
				; an error or it reached the
				; end of the vat
	ld de,_OP1		;where we want to store
				; name at
	call copy_to_op1	;copy backwards vat name to
				; op1 the right way
	ex de,hl		;we need it in hl so we
				; can display it
	call _putps		;display length indexed string
				; at hl

Using _FindAlphaUp/Dn

_FindAlphaUp=$514b

_FindAlphaDn=$514f

Speed is a huge drawback with these two; it's atleast 156,000 clock-cycles each time you call these which increases exponentially the deeper you are in the VAT. It's said to be the worst routine TI ever wrote; it's extremely slow! If you're only looking up one or two variables, OK. If you're making a shell that displays 30 variables, it could take upwards of 30 seconds!

	ld hl,beginning_var	;the variable name
				; to start searching up
				; from alphabetically
	rst 20h			;_mov10toOP1
	ld a,1			;ignore type
	call _FindAlphaUp	;find a variable higher
				; (closer to 'z') than the
				; one in OP1
				;hl=start of VAT entry
				;c - nothing found
				;nc - found one
				;OP1&3 - variable found
	ret c			;no such variable found
beginning_var:	.db $12,1,0	;0 is the single
				; character name
				;the lowest variable name
				; possible
				;$12=program type vari.

Shells

For a basic rendition of a shell, check out the demo program Shell.asm. It gives you all the basics to make your own shell for running Asm programs. You can easily add support for running TI-BASIC programs, displaying pictures, editing equations, editing programs, renaming variables, folders, etc.!