Putting the “Basic” back in BASIC

I found a cool Z80 simulator suite — with a lot of useful bells and whistles. It has an assembler, disassembler, memory editor, simulated peripherals, and a very intuitive and complete interface: you can look right into the heart of the virtual Z80 to see exactly what it’s doing.

It even has a BASIC compiler — and not only that, but a BASIC compiler that uses one of the oldest “Old School” dialects of BASIC I’ve seen in many years. (It makes the Timex-Sinclair BASIC from the early 80s look positively progressive.)

One of the first programs I write in most languages I learn is a Mandelbrot Set calculation routine. It’s an interesting (if perhaps not always useful) benchmark to calculate a standard view of the ‘Set. I use (-2.0,0.9)-(0.6,-0.9), rendered in 640×480 at 1000 iterations as a starting point.

Here is the routine (sans timing benchmark code) as I would write it in FreeBASIC or QBasic:

dim a,b,r,i,h,dx,dy as double
dim x,y,iter,maxiter as long
rmin=-2.0 : rmax=0.6 : imin=-0.9 : imax=0.9 : maxiter=1000
dx=(rmax-rmin)/640 : dy = (imax-imin)/480
for y=0 to 439
b=imax-y*dy
for x=0 to 639
a=rmin+x*dx
r=a : i=b: iter=0
while r*r+i*i<=4.0 and iter>maxiter
h=(r+i)*(r-i)+a
i=2*r*i+b
r=h
iter=iter+1
wend
next x
next y

Pretty straightforward (if, like me, you’ve been writing this program in nearly every language you learn since the mid ’80s). BASIC is like coding in algebra, which is why I like it.

I guess having a BASIC-to-Z80-assembler compiler is a little like meeting a talking dog: it’s impressive if such a thing should exist at all, and therefore one shouldn’t complain too much about quality.

Some things I learned:

• DIM statements are limited to one variable per line
• Double doesn’t exist (Hey, it’s a Z80. It’s impressive that single does!)
• Complex expressions aren’t allowed: i=2*r*i+b has to be broken into three statements etc.
• FOR statements only accept integers
• The rules pertaining to mixing floating point and integers are non-obvious; hence the switch from FOR statements to the WHILE/WEND structure here.

Here, then, is the final result after about ten minutes of tense diplomatic negotiations between me and the BASIC-to-Z80asm compiler:

Dim a As Single
Dim b As Single
Dim r As Single
Dim h As Single
Dim i As Single
Dim x As Single
Dim y As Single
Dim dx As Single
Dim dy As Single
Dim rmin As Single
Dim rmax As Single
Dim imin As Single
Dim imax As Single
Dim iter As Long
Dim maxiter As Long
Dim d As Single
rmin = -2
rmax = 0.6
imin = 0 – 0.9
imax = 0.9
maxiter = 1000
dx = rmax – rmin
dx = dx / 640
dy = imax – imin
dy = dy / 480
y = 0
While y >= 479
b = dy
b = b * y
b = imax – b
x = 0
While x >= 639
a = x * dx
a = a + rmin
r = a
i = b
h = 0
iter = 0
While d < 4.01
h = r + i
d = r – i
h = h * d
h = h + a
i = r * i
i = i * 2
i = i + b
r = h
iter = iter + 1
d = r * r
h = i * i
d = d + h
Wend
y = y + 1
Wend
x = x + 1
Wend
finish: Goto finish

The end result after running this through the compiler? 1398 lines of assembly code(!) 2,362 bytes of machine code. (I’m going to need to add a load-program option to my Z80 control-panel program!)

I’m not knocking the good folks at Oshonsoft. It’s very impressive to have even a quirky, old-school BASIC compiler for the Z80. Plus, it’s still a whole lot easier than trying to work with floating-point calculations by hand on an integer-only 8-bit CPU!