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HackDalton: CoolCPU 1

Warning! There are spoilers ahead

The two sample programs on the CoolCPU website help you understand the basics of how writing code works, so you should go through those two first.

So, from the description, we know that there's a string of characters at 0x93 that we want to print out. And, we know it's null-terminated (that is, it ends in a 0x00 bytes).

The basic idea for writing code to print the flag is to, starting at 0x93, read each byte and print it out, until we hit that 0x00. The instructions to do that might look something like this:

	CON 0x93				; 22 93
loop: 			; this will be at position 0x02
	LDB [A]					; 05
	SWB						; 40
	JZ end					; 31 0C
	SWB						; 40
	STB 0xF1				; 12 F1
	INC						; 20
	JP loop					; 30 02
end:			; this will be at position 0x0C
	HCF						; FF

The comments on the right show you the hexadecimal representations of each instruction.

Let's go through this step by step: first, the CON 0x93 sets A to 0x93, which is where we start our loop. We're using A to keep track of the current memory address.

Then, we start our actual loop. We use LDB [A] to read the value at A (the next character of our flag) and store it in register B.

Next up, we need to check if the byte we read was the 0x00 that signals the end. Unfortunately, we're only able to make comparisons with register A. So, we do SWB to swap registers A and B, JZ to check if it's zero, and then another SWB to bring things back to normal.

If it was zero, the JZ would jump to the HCF instruction that ends the program, and we're done. If it wasn't, then the STB 0xF1 writes the character to the output. Then the INC adds 1 to register A, and the JP loop goes back to the start of the loop so we can print the next character.

Combining the hexadecimal representations of each instruction, we get 22 93 05 40 31 0C 40 12 F1 20 30 02 FF. Running this code in the simulator, we get our flag!