Programming:Next / previous line calculation

Next line

This routines calculate the next line from the given address.

Firmware

Input: HL=Address

Output: HL=Address of the next line

Destroyed: AF

SCR_NEXT_LINE equ #BC26

call SCR_NEXT_LINE


ACTUAL #BC26 ROUTINE FOLLOWS:

ld a,h:add #08:ld h,a:and #38:ret nz
ld a,h:sub #40:ld h,a:ld a,l:add reg1*2:ld l,a:ret nc  ;reg1= CRTC reg.1 value
inc h:ld a,h:and #07:ret nz
ld a,h:sub #08:ld h,a:ret

Without Firmware

Input: HL=Address

Output: HL=Address of the next line

Destroyed: AF,BC

; Richard Fairhurst 
; October 1997 
; 
;*** Next line down from HL *** 

.nline  ld a,8 
        add h 
        ld h,a 
        ret nc
        ld bc,#c050
        add hl,bc 
        ret

Previous line

This routines calculate the previous line from the given address.

Firmware

Input: HL=Address

Output: HL=Address of the previous line

Destroyed: AF

SCR_PREV_LINE equ #BC29

call SCR_PREV_LINE


ACTUAL #BC29 ROUTINE FOLLOWS:

ld a,h:sub #08:ld h,a:and #38:cp #38:ret nz
ld a,h:add #40:ld h,a:ld a,l:sub reg1*2:ld l,a:ret nc ; reg1= CRTC reg.1 value
ld a,h:dec h:and #07:ret nz
ld a,h:add #08:ld h,a:ret

Fastest way to print a character in Mode 2

Input: HL=Address of sprite/char.data DE=screen address (works for every first line of screen !! For example: &C000, &C050, &C0A0 etc..)

Destroyed: AF, C

Unchanged: DE returns to its input value (through C register)

'If HL is not page aligned, INC HL must be used instead of INC L (1 NOP SLOWER!)

ld c,d
ld a,(hl):ld(de),a:inc l:set 3,d
ld a,(hl):ld(de),a:inc l:ld d,c:set 4,d
ld a,(hl):ld(de),a:inc l:set 3,d
ld a,(hl):ld(de),a:inc l:ld d,c:set 5,d
ld a,(hl):ld(de),a:inc l:set 3,d
ld a,(hl):ld(de),a:inc l:set 4,d:res 3,d
ld a,(hl):ld(de),a:inc l:set 3,d
ld a,(hl):ld(de),a
ld d,c