The common way to explain the layout of the ZX Spectrum's pixel orientation on its bitmapped VRAM is often quite convoluted and is oriented towards the values of each bit of the VRAM address - useful for plotting single pixels, but not for batch operations.
The Speccy VRAM can be visualized in a few ways to help understand how it's laid out:
1) Similar to an MSX, the ZX has 3 sets of 256x8x8 blocks arranged in a 32x24 grid. From $4000-$47ff is the first set, $4800-$4fff is the second, and $5000-$57ff is the third.
2) Pixel data is oriented in VRAM as if it were a 2048x24 bitmap (with each byte representing 8 pixels for 256x24 bytes), then the 8x8 tiles were scrunched into 256x192.
ONE PIXEL DOWN:
Add 1 to H, every 8 add 32 to L and reset H.
(if L rolls over, add 8 to H.)
EIGHT PIXELS RIGHT:
Add 1 to L.
This layout can do a couple things with the target VRAM address:
1. inc l will increase the pixel X position across 8 rows (256 bytes per page / 32 columns = 8 rows)
2. inc h will increase the pixel Y position within the first 8 rows, plus the row offset from the l register.
3. Flooding VRAM with patterns is really easy and fast:
ld hl, $4000 ; VRAM base
ld b, 12 ; 2 rows per loop * 12 = 24 rows
ld a, %01010101 ; pixel pattern row 1
ld [hl], a
jr nz, .loop_a
ld a, %10101010 ; pixel pattern row 2
ld [hl], a
jr nz, .loop_b
jr nz, .printloop