Reaching the machine

BASIC is a program running on a 65C02, and it doesn't hide the fact. Six keywords let you reach past it.

Looking at memory

PEEK(address) reads one byte, 0 to 255. POKE address, value writes one:

POKE 2560, 42
PRINT PEEK(2560)

 42

OK

Addresses run from 0 to 65535. That's the whole map — RAM, the ROM that BASIC itself lives in, and the hardware.

Poking at random

Some of that memory is BASIC's own working space, and writing to it will stop the machine mid-sentence. It can't damage anything: press reset, or switch off and on, and you're back. But whatever you'd typed is gone, so SAVE first.

Finding out what's fitted

At switch-on the machine looks for every card it knows about and records what it found in one byte, at address 781. Each bit is one card:

Bit	Value	Card
7	128	video
6	64	sound
5	32	keyboard and joysticks
4	16	serial
3	8	storage
2	4	clock
1	2	banked RAM, upper half
0	1	banked RAM, lower half

10 H = PEEK(781)
20 PRINT "THIS ACE HAS"
30 PRINT
40 IF H AND 128 THEN PRINT "VIDEO"
50 IF H AND 64 THEN PRINT "SOUND"
60 IF H AND 32 THEN PRINT "KEYBOARD AND JOYSTICKS"
70 IF H AND 16 THEN PRINT "SERIAL"
80 IF H AND 8 THEN PRINT "STORAGE"
90 IF H AND 4 THEN PRINT "CLOCK"
100 IF H AND 3 THEN PRINT "BANKED RAM"
110 PRINT
120 PRINT "FREE MEMORY"; FRE(0)

On an ACE, that program prints everything on the list — an ACE has the lot. It earns its keep on a machine you built yourself, and as the polite way for a program to check before it uses something:

10 IF (PEEK(781) AND 64) = 0 THEN PRINT "NO SOUND CARD - PLAYING QUIETLY"

MEM prints the same byte in hex, along with the free memory and the current disk, which is quicker to type when you just want a look:

MEM

 30718 BYTES FREE  HW=$FF
DISK 0

OK

How much room is left

FRE(0) gives you the bytes available for variables, arrays and strings:

PRINT FRE(0)

 30718

OK

That's a freshly-started machine. Every program line, variable and array comes out of it. The argument to FRE is ignored — FRE(0) is just how it's written.

The extra RAM

Above the ordinary memory there's a window at address 32768 backed by banked RAM, and BANK n chooses which bank shows through it:

BANK 1
POKE 32768, 42
BANK 0
PRINT PEEK(32768)
BANK 1
PRINT PEEK(32768)

 0
 42

OK

Same address, different bank, different byte. It's how you keep more data than fits in one go — a map, a screen, a level — and swap between them.

Calling machine code

SYS address calls machine code and comes back when it returns:

POKE 2560, 96
SYS 2560

96 is the byte for RTS, "return from subroutine", so that program calls one instruction that does nothing but come straight back. It's not useful; it's the smallest possible demonstration that the door is there.

What's on the other side of it is machine code you wrote yourself, and SYS is how BASIC hands over to it.

Straight into the Monitor

BRK drops you out of BASIC and into the machine-code Monitor:

BRK

6502 MONITOR v1.1
BRK AT $E9D1
PC=E9D3 A=00 X=FF Y=68 SP=FA ---B-IZC
.

The . is the Monitor's prompt. X there brings you back to BASIC with your program still in memory. The Monitor is the tour.

Next: what to do when none of this works.