Retrogramming

Getting back to machine-language, we will take the program from a previous post , assemble it by hand, and create a PRG file that can be run on a C64 emulator.  The program is a very simple one that changes the background and border color to black: LDA #00 STA $D020 STA $D021 RTS If we look up the opcodes by hand or use an assembler, we can see that the machine language code for this program is the following byte sequence: A9 00 8D 20 D0 8D 21 D0 60 One way to run this program, at least on a Commodore 64 emulator, is to create a PRG file with it. Another way would be to create a disk image (D64). But the PRG format is much simpler, so we go with it, for now. A PRG file consists of a 2-byte header followed by the binary contents of the program to load into memory. The header specifies the memory address for the binary code to be loaded in the C64 memory space. So we could start with the bytes 00 C0 (the 6502 is a little-endian CPU) and load the code to address $C000; then we sho...

 As it happens with older computers, with the C64 the programmer had complete control over the machine and could write directly to any part of memory, perform I/O and so on. Video memory was mapped to the CPU address space and so could be read and written directly as well.  For the C64, the relevant memory addresses are 0x0400 for characters and 0xD800 for colors. The byte at 0x0400 determines the character that appears at the top left corner at the screen, while the byte at 0xD800 determines the color of this character. The following bytes store the next characters in order going from left to right, and top to bottom. So, to find the address of screen position (x, y) for column x and row y, you have to calculate 40*y+x. Multiplication can be slow in these old CPUs, but we can fill the memory sequentially and, en passant, see the complete character set of the computer (it does not follow ASCII).  So here's a small BASIC program to manipulate video memory using POKEs. We c...

There are many books, tutorials etc dedicated to teaching C64 programming, many of them start by teaching BASIC. I had an MSX as a kid so I'm pretty familiar with this flavor of BASIC (Microsoft BASIC). On the other hand, most assembly programming tutorials begin with number systems, binary numbers and other pretty basic stuff for someone who's already familiar with assembly programming in other platforms.  What I wanted was to create the smallest programs in assembly that could run on the machine and show some effect. After looking around for a bit I've found this one: lda #$00 sta $d020 sta $d021 rts This sets the contents of the accumulator to 0 and then store this zero at memory locations 0xD020 and 0xD021. These two memory locations control the color of the screen background (0xD021) and the border (0xD020). As expected, setting them to zero will make the whole screen black (except for the text). Now the next step is how to run this program. The id...