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fix bug (first char XOR with initial state)

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This commit is contained in:
Arthur Lu 2022-08-08 22:21:03 -07:00
parent f3bcd0e772
commit 1c8fcec85b
1 changed files with 9 additions and 9 deletions
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18
program1.asm
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@ -24,12 +24,12 @@ init: LDI #d62
PUT r8 // init r8 decrementer with number of preamble space chars
LDI #d64
PUT r9 // init r9 decrementer 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
preamble_loop:LDI #d32 // get space character decimal 32
XOR r7 // 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
LDI #lfsr_routine // load address for the lfsr_routine label
JAL r0 // jump to the lfsr_routine label
NXT r12 // increment write pointer
NXT r9 // decrement number of remaining ciphertext characters
LDI #main_loop // load the address of label main_loop
@ -37,12 +37,12 @@ preamble_loop: LDI #lfsr_routine // load address for the lfsr_routine label
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
main_loop: LDW r11 // load the next plaintext byte
XOR r7 // 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
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
@ -57,7 +57,7 @@ lfsr_routine: GET r7 // get previous state
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
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