Can someone help explain the following code. I'm trying to add comments to the following SIC/XE program that explains how this program is calculating a fibonacci sequence for the first 20 numbers, starting at 3.
FIB START 0
First LDT #3 ;start at number 3
LDS #20 ;calculate up to 20
+JSUB RDREC ;call read subroutine
+JSUB WRREC ;call write subroutine
LENGTH RESW 1
.
. SUBROUTINE RDREC
.
RDREC CLEAR A ;set A to zero
LOOP1 TD INPUT ;Test input device
JEQ LOOP1 ;Loop if device is busy
RD INPUT ;read one byte into register A
COMPR A,T ;Test for end
JLT LOOP1 ;jump to loop until end
COMPR A,S ;
JGT LOOP1
STA LENGTH
RSUB
INPUT BYTE X'F1'
.
.SUBROUTINE WDREC
.
WRREC LDX #0
LDS #1
LDA #0
LDT LENGTH
LOOP2 TD OUTPUT
JEQ LOOP2
WD OUTPUT
ADDR S,A
STA VALUE1
STS VALUE2
LDA VALUE2
LDS VALUE1
TIXR T
JLT LOOP2
OUTPUT BYTE X'05'
VALUE1 RESW 1
VALUE2 RESW 1
END FIRST
