Tables and Arrays

z80 » Intermediate

A lot of times you want to access info in a table or a list. Remember the index registers?

	ld iy,table_start
	ld a,(iy+4)
You would be referencing the fourth item from the beginning of the list with the address of iy's value. You still retain the address for the beginning of the table.

That's good for accessing specific elements in the list. If you want to take a list and check each thing against a value, the best thing is to use a recursive loop. Here's a simple routine to check what's in a in comparison to every item in the table. If there's a match, the place value of that match is put into b; if there's no match, b is loaded with 0:

	ld b,end_of_table-table
	ld hl,table
	cp (hl)
	ret z
	inc hl
	djnz check_loop
table:  .db $34,$a3,$eb,$56,4,1,$20,$ff,220,$0a

Here's some different code that does the same thing but is shorter and uses a new instruction (cpir) which does the following: compares a to (hl), if it's equal then exit, if not decrease bc and repeat until either bc is zero or a=(hl). Here is the same routine from above, but instead it uses cpir:

	ld bc,end_of_table-table
	ld hl,table
table:  .db $34,$a3,$eb,$56,4,1,$20,$ff,220,$0a

The second routine is two bytes shorter. Two bytes doesn't seem like anything really but it adds up and everything. Sometimes the only difference between two versions of a game is a five or six byte change!

Let's say you wanted to make an array (list) with each element 1 byte in length to hold the letters of the alphabet. Let's also say the array can be trashed once your program is done. We'll use a safe memory area at the end of our program at, say, address $f200. Let's start loading the array with letter values, incrementing the array length counter (array_size at address $f200). We already know that we only have 26 letters but we're using a counter to demonstrate the principle of having some counter to tell you the size of the array. The actual array's element data will start at address $f201. You can just download the source code if you want to assemble it to see the results.

Keep in mind, this is a really cumbersome method of referencing elements. I'm only trying to show you the fundamentals. It would be much faster to do it as shown in Array2.asm with Simulated 16 bit Addition.

a_size		=$f200	;array size
array_start	=$f201	;start of element data
e_size		=1	;element size in bytes
#define array(element)	array_start+((element-1)*e_size)
	;each element is two bytes
	;when we want the 2nd element, we usually
	; put 'array(2)' but since 0 is the first
	; element, we subtract 1 from what we
	; requested
#include ""
.org _asm_exec_ram

;let's start loading the array with
; values
	ld hl,array(1)	;return the address of the
			; first element in de
	ld ix,a_size	;size/length counter address
	ld (ix),$00	;initialize counter to $00
			; because we haven't stored
			; anything in it yet
			;ix register pair can be used
			; just like hl
	ld de,e_size	;load de with the size
			; of each element so we
			; can move from one element
			; to the next
	ld b,26		;there are 26 characters
			; in the alphabet...i hope
	ld a,'a'	;letter to start with
	ld (hl),a	;load element
	inc a		;increment letter value
	inc (ix)	;increment counter value
	add hl,de	;add where we are to
			; the length of each element
			; to get the next element's
			; address
	djnz initialize_loop

	call _clrScrn	;clear the screen
	call _homeup	;put cursor at top left

;now let's print the word "stupid" letter by letter

	ld hl,array(19)		;'s'
	call print_element

	ld hl,array(20)		;'t'
	call print_element

	ld hl,array(21)		;'u'
	call print_element

	ld hl,array(16)		;'p'
	call print_element

	ld hl,array(9)		;'i'
	call print_element

	ld hl,array(4)		;'d'
	call print_element

	call _getkey	;wait for keypress
	jp _clrScrn	;clear screen and return
;print_element prints the character in the
; array at address held by hl
	ld a,(hl)	;get that element's value
	jp _putc	;print character and return

To get a little more complicated, let's check out tables. Here's a look up routine for a table with 3 columns. You input into d the desired row, e the desired column, and have hl hold the address of the queried table. You can see this routine in action used by Table.asm. It also uses Simulated 16 bit Addition.

	ld de,1*256+2	;row 1, col 2
	ld hl,table
	call get_cell
	. . .
	. . .
;input: d=row
;	e=col
;	hl=address of 3x3 table
	dec e		;decrease columns
			; since $00 is 1st
			; column
	dec d		;decrease rows
			; since $00 is 1st
			; column
	sub a		;initialize to $00
	ld b,d		;get row #
	jr z,no_row_mult
	add a,table_width	;width of table (3)
	djnz row_multiply_loop
	add a,e		;add in col #
;now we get into simulated 16 bit addition
; of adding an 8 bit register (a) to a 16 bit one (hl)
	add a,l
	ld l,a
	adc a,h
	sub l
	ld h,a
	ld a,(hl)
	.db 'a','b','c'
	.db 'd','e','f'
	.db 'g','h','i'

This is still only an introduction, designed just to teach you the basics. There are so many other ways to use tables and arrays that you will have to discover on your own and tailor to your programs.

More from z80 » Intermediate
All the Flags // Debugging // Down-Left Bug // _GetKey Codes // Logical Operators // Memory, ROM, RAM, and Safe Areas // Miscellaneous Instructions // PC and SP // Random Numbers // TI's ROM Calls // Restart Commands // Simulated 16-bit Addition // The Stack // Tables and Arrays // Text Display // Variables