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How to convert a zx spectrum, x, y coordinate to a screen address via a lookup table. See http://www.overtakenbyevents.com/lets-talk-about-the-zx-specrum-screen-layout-part-three/
; IN - B = pixel row (0..191)
; IN - C = character column (0..31)
; OUT - HL = screen address
; OUT - DE = trash
coords_to_address:
ld h, 0
ld l, b ; hl = row
add hl, hl ; hl = row number * 2
ld de, screen_map ; de = screen map
add hl, de ; de = screen_map + (row * 2)
ld a, (hl) ; implements ld hl, (hl)
inc hl
ld h, (hl)
ld l, a ; hl = address of first pixel in screen map
ld d, 0
ld e, c ; de = X (character based)
add hl, de ; hl = screen addr + 32
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
@SimonGreenaway
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SimonGreenaway commented Apr 18, 2021

Line 10: add hl, de ; de = screen_map + (row * 2)
Should be: add hl, de ; hl = screen_map + (row * 2)

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