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cse141L-project/firmware/program2.asm

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// Program 2 register use map:
// r0 is the accumulator, r1 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
LDI done
NXT r10 // decrement tap selection by 1, starts at 9 for the first iteration
JEZ r0 // 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: LDW r11 // load the next ciphertext byte
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
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 #d1 // 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
LDI #d255
JMP r0