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add program1.asm

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Arthur Lu 2022-08-07 16:18:56 -07:00 committed by GitHub
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// Program 1 register use map:
// r0 is the accumulator, r1 is often used to cache temp values
// r6 is LFSR tap pattern
// r7 is LFSR state value
// r8 is the preamble counter
// r9 is the total encryption length counter
// r11 is the read pointer
// r12 is the write pointer
init: LDI #d62
PUT r8
LDW r8
PUT r6 // tap will now be in r6
LDI #d63
PUT r8
LDW r8
PUT r7 // state will now be in r7
LDI #d0
PUT r11 // init read incrementor to 0
LDI #d64
PUT r12 // init write incrementor to 64
LDI #d61
PUT r8
LDW r8
PUT r8 // init r8 decrementor with number of preamble space chars
LDI #d64
PUT r9 // init r9 decrementor to total number of possible ciphertext chars
preamble_loop: LDI #lfsr_routine // load address for the lfsr_routine label
JAL r0 // jump to the lfsr_routine label
LDI #d32 // get space character decimal 32
XOR r1 // bitwise XOR the current state with plaintext space to generate ciphertext
CLB r0 // clear the leading bit of the ciphertext as in requirements
STW r12 // store ciphertext to write pointer
NXT r12 // increment write pointer
NXT r9 // decrement number of remaining ciphertext characters
LDI #main_loop // load the address of label main_loop
NXT r8 // decrement preamble counter
JEZ r0 // exit preamble loop if the preamble counter has just reached 0
LDI #preamble_loop // load the address of label preamble_loop
JMP r0 // jump to preamble_loop if there are more space characters to encode
main_loop: LDI #lfsr_routine // load address for the lfsr_routine label
JAL r0 // jump to the lfsr_routine label
LDW r11 // load the next plaintext byte
XOR r1 // bitwise XOR the current state with plaintext space to generate ciphertext
CLB r0 // clear the leading bit of the ciphertext as in requirements
STW r12 // store ciphertext to write pointer
NXT r11 // increment read pointer
NXT r12 // increment write pointer
LDI #done // load address of label done
NXT r9 // decrement number of remaining ciphertext chars
JEZ r0 // jump to end of program if all ciphertext 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 r1 // put next state to r1
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