Draw with a counted loop

Our last program drew eight rows with eight pairs of instructions. We can give that repeated job to a loop: a piece of code that branches back to do another turn. The picture stays the same, but its eight patterns now live together as data.

If you made your own character in lesson 3, run it there and press Keep this character for the loop lesson. Return here in the same tab and choose Use my saved character. This copies its eight bytes into the data table. The supplied picture works too.

Three things to keep track of

A loop needs to know where to read, where to write, and when to stop.

HL and DE are register pairs. Each combines two 8-bit registers into a 16-bit value, large enough to hold an address. H and D are the high bytes; L and E are the low bytes. With DE equal to $4000, D holds $40 and E holds $00.

ld hl,patterns loads an address into HL. It does not read the byte at that address. ld a,(hl) does the read: parentheses mean the contents of memory, as in the previous lessons.

Follow one turn

row:
 ld a,(hl)
 ld (de),a
 inc hl
 inc d
 djnz row

The first instruction reads the pattern pointed to by HL into A. The second copies A into the memory byte pointed to by DE.

inc hl adds one to the source address, selecting the next byte in the table. inc d adds one to the high byte of the destination: $4000 becomes $4100, then $4200, and so on. E stays zero. These are different steps because the table is stored consecutively while the screen rows are not.

djnz row means decrement B and jump if not zero. It subtracts one from B, then branches to row if the new value is non-zero. After the eighth write, B reaches zero and execution continues into the hold loop.

Run it, then replay the evidence

Run the program, then press Next write to reveal each captured write. The Spectrum completes this tiny loop much faster than we can follow by eye. The replay shows writes recorded during real execution; it does not slow the machine, execute another loop, or simulate what the source ought to do.

The live picture and memory table show the finished run. The separate replay lets you examine the writes that produced it. Try the count and address experiments before changing the data.

; Copy eight pattern bytes into the top-left character cell.
 org 32768
start:
 ld a,0
 out ($FE),a
 ld a,71
 ld ($5800),a
 ld hl,patterns
 ld de,$4000
 ld b,8
row:
 ld a,(hl)
 ld (de),a
 inc hl
 inc d
 djnz row
hold:
 jr hold
patterns:
 defb %00011000
 defb %00111100
 defb %01111110
 defb %11011011
 defb %11111111
 defb %00111100
 defb %01100110
 defb %01000010
 end start

The count is part of the drawing rule

With ld b,4, the program writes four rows. It does not clear the four rows it never touches. With inc d removed, it writes eight different patterns to the same address: the last one replaces the others.

Keep the count between 1 and 8 for these experiments. The table has eight bytes and the destination step is designed for one character cell. More turns read beyond that table and write elsewhere on the screen. Starting B at zero does not skip the body: the body runs first, then DJNZ wraps zero to 255. That gives 256 turns before B finally returns to zero.

Keep the data, reuse the method

The eight defb lines are your artwork. You can replace them without changing the drawing instructions. This is a useful separation: the data describes what to draw, and the loop describes how to place it.

This first loop handles only the top-left 8×8 cell. Its source pointer advances through a table one byte at a time; its screen pointer follows the Spectrum’s layout. Drawing elsewhere will require us to choose a suitable starting address and respect the layout’s boundaries.

Use Download your source to keep row-loop.asm. For a local build:

asm198x --dialect pasmo --cpu z80 --tapbas row-loop.asm -o row-loop.tap

Or, with Pasmo:

pasmo --tapbas row-loop.asm row-loop.tap

Load the tape in a fresh 48K Spectrum as in lesson 1. Check the eight rows against the previous program. The browser’s recorded-write replay is a teaching tool; the downloaded program is ordinary Z80 code and needs no replay controls to draw.