How to convert a zx spectrum, x, y coordinate to a screen address via a lookup table. Forked from David Black's version, shorter, does not trash DE. The modification is based on the fact that character offset maximum value is 0x1F, which means we don't need to use 16-bit addition to add up the X coordinate.

space.z80

; IN  -   B = pixel row (0..191)
; IN  -   C = character column (0..31)
; OUT -  HL = screen address
; DE - unaffected

coords_to_address:  
    ld	a, c
    ld  h, 0
    ld  l, b            ; hl = row
    add hl, hl          ; hl = row number * 2
    ld  bc, screen_map  ; de = screen map
    add hl, bc          ; de = screen_map + (row * 2)
    add a,(hl)
    inc hl
    ld  h, (hl)         
    ld l,a
    ret                 ; return screen_map[pixel_row]

screen_map:		
	.defw #4000, #4100, #4200,  #4300 
	.defw #4400, #4500, #4600,  #4700 
	.defw #4020, #4120, #4220,  #4320 
	.defw #4420, #4520, #4620,  #4720 
	.defw #4040, #4140, #4240,  #4340 
	.defw #4440, #4540, #4640,  #4740 
	.defw #4060, #4160, #4260,  #4360 
	.defw #4460, #4560, #4660,  #4760 
	.defw #4080, #4180, #4280,  #4380 
	.defw #4480, #4580, #4680,  #4780 
	.defw #40A0, #41A0, #42A0,  #43A0 
	.defw #44A0, #45A0, #46A0,  #47A0 
	.defw #40C0, #41C0, #42C0,  #43C0 
	.defw #44C0, #45C0, #46C0,  #47C0 
	.defw #40E0, #41E0, #42E0,  #43E0 
	.defw #44E0, #45E0, #46E0,  #47E0 
	.defw #4800, #4900, #4A00,  #4B00 
	.defw #4C00, #4D00, #4E00,  #4F00 
	.defw #4820, #4920, #4A20,  #4B20 
	.defw #4C20, #4D20, #4E20,  #4F20 
	.defw #4840, #4940, #4A40,  #4B40 
	.defw #4C40, #4D40, #4E40,  #4F40 
	.defw #4860, #4960, #4A60,  #4B60 
	.defw #4C60, #4D60, #4E60,  #4F60 
	.defw #4880, #4980, #4A80,  #4B80 
	.defw #4C80, #4D80, #4E80,  #4F80 
	.defw #48A0, #49A0, #4AA0,  #4BA0 
	.defw #4CA0, #4DA0, #4EA0,  #4FA0 
	.defw #48C0, #49C0, #4AC0,  #4BC0 
	.defw #4CC0, #4DC0, #4EC0,  #4FC0 
	.defw #48E0, #49E0, #4AE0,  #4BE0 
	.defw #4CE0, #4DE0, #4EE0,  #4FE0 
	.defw #5000, #5100, #5200,  #5300 
	.defw #5400, #5500, #5600,  #5700 
	.defw #5020, #5120, #5220,  #5320 
	.defw #5420, #5520, #5620,  #5720 
	.defw #5040, #5140, #5240,  #5340 
	.defw #5440, #5540, #5640,  #5740 
	.defw #5060, #5160, #5260,  #5360 
	.defw #5460, #5560, #5660,  #5760 
	.defw #5080, #5180, #5280,  #5380 
	.defw #5480, #5580, #5680,  #5780 
	.defw #50A0, #51A0, #52A0,  #53A0 
	.defw #54A0, #55A0, #56A0,  #57A0 
	.defw #50C0, #51C0, #52C0,  #53C0 
	.defw #54C0, #55C0, #56C0,  #57C0 
	.defw #50E0, #51E0, #52E0,  #53E0 
	.defw #54E0, #55E0, #56E0,  #57E0