implement program3,

fix indentation in program2

program3.asm

@@ -0,0 +1,112 @@
+// Program 2 register use map:
+// r0 is the accumulator, r1 and r2 is often used to cache temp values
+// r5 is the TAP LUT link register
+// r6 is LFSR tap pattern
+// r7 is LFSR state value
+// r8 is the preamble counter
+// r9 is the total encryption length counter
+// r10 is the tap selection counter
+// r11 is the read pointer
+// r12 is the write pointer
+init: LDI #d10
+	PUT r10 // set the tap counter to 10, which will choose tap pattern 9 to start after subtracting by 1
+tap_lut: LDI #tap_init
+	JMP r0 // goto tap_init, skipping the LUT
+	LDI #x60 // load tap pattern 1
+	JMP r5 // jump back to tap loop
+	LDI #x48 // load tap pattern 2
+	JMP r5 // jump back to tap loop
+	LDI #x78 // load tap pattern 3
+	JMP r5 // jump back to tap loop
+	LDI #x72 // load tap pattern 4
+	JMP r5 // jump back to tap loop
+	LDI #x6A // load tap pattern 5
+	JMP r5 // jump back to tap loop
+	LDI #x69 // load tap pattern 6
+	JMP r5 // jump back to tap loop
+	LDI #x5C // load tap pattern 7
+	JMP r5 // jump back to tap loop
+	LDI #x7E // load tap pattern 8
+	JMP r5 // jump back to tap loop
+	LDI #x7B // load tap pattern 9
+	JMP r5 // jump back to tap loop
+tap_init: LDI #d64
+	PUT r11 // set read pointer to 64
+	LDI #d0 
+	PUT r12 // set write pointer to 0
+	LDI #d10
+	PUT r8 // load 10 into preamble counter
+	LDI #d64
+	PUT r9 // load 64 (total encryption length) to r9
+	NXT r10 // decrement tap selection by 1, starts at 9 for the first iteration
+	JEZ #done // if no more taps left that didn't work, raise the done flag
+	LDI #tap_init
+	PUT r5 // put the tap_loop address in r5
+	LDI #tap_lut
+	ADD r10
+	ADD r10 // add 2*tap select to tap_lut location, results in location of selected tap pattern
+	JMP r0 // jump to LUT, which loads the tap pattern into r0
+	PUT r6 // tap pattern now in r6
+	LDW r11 // get the first preamble character
+	PUT r1 // put cipher text into r1
+	LDI #d32 // load expected space character
+	STW r12 // write initial space into memory
+	XOR r1 // get the initial state
+	PUT r7 // put initial state guess into r7
+	NXT r11 // increment read pointer
+	NXT r12 // increment write pointer
+	NXT r9 // decrement total encryption chars remaining
+	tap_loop: LDI #lfsr_routine
+		JAL r0 // jump to lfsr routine which calculates next state in r7
+		LDI #d32 // load space char expected plaintext
+		XOR r7
+		CLB r0 // clear leading bit in the expected ciphertext
+		PUT r1 // store expected cipher text in r1
+		LDI #tap_init 
+		PUT r2 // load the outer loop top into r2
+		LDW r11 // load actual ciphertext
+		SUB r1 // subtract actual from expected, result of 0 means matching
+		JNZ r2 // jump to outer loop (picks new tap pattern) if the actual cipher was not equal to the expected
+		LDI #d32 // load preamble char
+		STW r12 // store preamble char in memory
+		NXT r11 // increment read pointer
+		NXT r12 // increment write pointer
+		NXT r9 // decrement total encryption chars remaining
+		LDI #main_loop // load main_loop location into r0
+		NXT r8 // decrement preamble counter
+		JEZ r0 // if r8 (preamble counter) is zero, then all preamble have matched and current tap pattern is correct, jump to main loop
+		LDI #tap_loop
+		JMP r0 // jump to tap_loop if characters matched but preamble is not over
+main_loop: LDI #correct
+	PUT r1 // put correct handle address in r1
+	LDW r11 // load the next ciphertext byte
+	CHK r0 // check ciphertext for error
+	JEZ r1 // if no error, jump to correct, otherwise continue to error handling
+	error: LDI #x80 // load error flag character into r0
+	STW r12 // store error flag to write pointer
+	LDI #common
+	JMP r0 // jump out of error handling, to common operations after writing
+	correct: XOR r7 // bitwise XOR the current state with ciphertext space to generate plaintext
+	CLB r0 // clear the leading bit of the plaintext as in requirements
+	STW r12 // store plaintext to write pointer
+	common: LDI #lfsr_routine // load address for the lfsr_routine label
+	JAL r0 // jump to the lfsr_routine label
+	NXT r11 // increment read pointer
+	NXT r12 // increment write pointer
+	LDI #done // load address of label done
+	NXT r9 // decrement number of remaining plaintext chars
+	JEZ r0 // jump to end of program if all plaintext chars have been processed
+	LDI #main_loop // load address of main_loop
+	JMP r0 // jump to main_loop if there is still space for message characters
+lfsr_routine: GET r7 // get previous state
+	AND r6 // and state with taps to get feedback pattern
+	PTY r0 // get feedback parity bit
+	PUT r1 // store feedback bit to r1 temporarily
+	GET r7 // get previous state again
+	LSH #1 // left shift previous state by 1
+	ORR r1 // or with parity bit to get next state
+	PUT r7 // put next state to r7
+	GET r14 // load link register
+	JMP r0 // return to function call address
+done: LDI #b10000000 // load the processor flag state needed to halt the program
+	PUT r15 // put and set the done flag to 1 to halt the PC and indicate the program has finished