I've been messing around with the ZX Spin emulator and Assembly language. I used to write BASIC games back in the 80s, and always wanted to learn machine code, but too stupid as a 12 year old kid. I've been putting this simple code together that simply creates a series of UDGs that make up an image, and should print the image onto the screen. I'm a little stuck though on how to get it to use my TIT_X_POS and TIT_Y_POS variables, which I have stored and initialised into memory locations. It seems to be tied to the "_AT" coordinates in the DEFB statements, and you cannot seem to pass in variables into those. Does anyone have any quick tips? Here is the code:
ORG 30000
_MAIN
CALL SUB_STARTUP
CALL SUB_PRINT_TIT ;Print TIT sprite
RET ;Return to BASIC
_SUBS
SUB_STARTUP
CALL CLS
LD A, 0
LD (TIT_Y_POS), A
LD A, 25
LD (TIT_X_POS), A
RET
SUB_PRINT_TIT
LD HL, TIT_UDGS ;Get location of UDG data.
LD (UDGPOINTER), HL ;Set UDG SYSVAR pointer.
LD A, 2 ;Set output channel 2 (upper screen).
CALL OUTCHAN ;Activate output channel.
LD DE, TIT_STR ;Start of string.
LD BC, TIT_STR_END-TIT_STR ;Length of string.
;;;;;;;;;;STUCK HERE - How do I get it to use TIT_Y_POS and TIT_X_POS instead of the _AT values in DEFB?
CALL PRINTSTRING ;Print string.
RET ; Return from SUB_PRINT_TIT
_SYSVARS
UDGPOINTER EQU 23675 ;UDG pointer.
TIT_X_POS EQU 23700 ;Memory location for X coordinate
TIT_Y_POS EQU 23701 ;Memory location for Y coordinate
_ROM
BORDER EQU 8859 ;Set border color.
CLS EQU 3503 ;Clear screen using attributes in ATTR-P (23693)
OUTCHAN EQU 5633 ;Activate output channel.
PRINTNUMSMALL EQU 6683 ;Print number in BC (9999 max).
PRINTNUMBIG1 EQU 11563 ;(1 of 2) to print number in BC (65535 max).
PRINTNUMBIG2 EQU 11747 ;(2 of 2) to print number in BC (65535 max).
PRINTSTRING EQU 8252 ;Print string (DE start, BC length).
_VALUES
;Store values into memory here.
_STRINGS
TIT_STR ;TIT sprite string.
DEFB _AT, 0, 0, ink, blk, paper, wht, flash, off, bright, off, ua, ub, ue, uf ;TIT - top
DEFB _AT, 1, 0,ink, blk, paper, wht, flash, off, bright, off, uc, ud, ug, uh ;TIT - bottom
TIT_STR_END EQU $ ;Marker for end of string data.
_CODES EQU $
cr equ 13 ;carriage return.
ink equ 16 ;INK control code.
paper equ 17 ;PAPER control code.
flash equ 18 ;FLASH control code.
bright equ 19 ;BRIGHT control code.
inverse equ 20 ;INVERSE control code.
off equ 0 ;OFF control code.
on equ 1 ;ON control code.
_at equ 22 ;AT control code.
_tab equ 23 ;TAB
blk equ 0 ;Blk.
blu equ 1 ;Blue.
red equ 2 ;Red.
mag equ 3 ;Magenta.
grn equ 4 ;Green.
cyn equ 5 ;Cyan.
yel equ 6 ;Yellow.
wht equ 7 ;White.
ua equ 144 ;UDG "A".
ub equ 145 ;UDG "B".
uc equ 146 ;UDG "C".
ud equ 147 ;UDG "D".
ue equ 148 ;UDG "E".
uf equ 149 ;UDG "F".
ug equ 150 ;UDG "G".
uh equ 151 ;UDG "H".
ui equ 152 ;UDG "I".
uj equ 153 ;UDG "J".
uk equ 154 ;UDG "K".
ul equ 155 ;UDG "L".
um equ 156 ;UDG "M".
un equ 157 ;UDG "N".
uo equ 158 ;UDG "O".
up equ 159 ;UDG "P".
uq equ 160 ;UDG "Q".
ur equ 161 ;UDG "R".
us equ 162 ;UDG "S".
ut equ 163 ;UDG "Y".
uu equ 164 ;UDG "U".
_UDGS ;********************************* UDGS ********************************
TIT_UDGS ;UDGs.
; (A,B)Top-left TIT sprite
DEFB 15,18,33,41,41,41,33,17, 255,2,1,0,32,16,16,32
; (C,D)Bot-left TIT sprite
DEFB 18,13,0,0,0,1,0,0, 0,255,64,64,64,255,0,0
; (E,F)Top-right TIT sprite
DEFB 255,32,144,72,72,48,0,0, 255,1,2,2,4,8,16,16
; (G,H)Bot-right TIT sprite
DEFB 0,255,2,2,2,255,0,0, 32,192,0,0,0,128,0,0
Many thanks in advance.
I discovered that the easiest solution is to just dynamically adjust the bytes stored in the "TIT_STR" itself with the values of the X/Y coordinates in there relevant places according to the DEFBs:
Therefore, as long as "TIT_Y_POS" and "TIT_X_POS" are updated prior, simply calling "SUB_TIT_PRINT" will always honour the locations according to the coordinates.