Create a demo program to check if values exist

2026-04-19 21:39:45 +02:00
parent e5700ddc95
commit 7439e26a69
1 changed files with 43 additions and 0 deletions
--- a/src/chkmat.s
+++ b/src/chkmat.s
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+;-*-MIDAS-*-
+        TITLE    CHECK MATRIX
+A=1                                    ;Define the ACs as universal registers
+B=2                                    ;from A to E
+C=3
+D=4
+E=5
+X=10                                   ;X and Y are used for indexing
+Y=11                                   ;loops etc.
+P=17                                   ;Define P as the stack pointer
+
+PDLLEN==30                             ;Set the size of the stack to 30 words.
+PDL:        BLOCK PDLLEN               ;Reserve the stack memory
+
+START:  MOVE  P,[-PDLLEN,,PDL-1]       ;Initializes the stack
+DONE:   .LOGOUT 2,                     ;Exit from the program
+
+;Subroutine that checks a byte array, where one byte is a 4-bit value,
+;for duplicate values.
+;The algorithm works as follows:
+;The original word is walked through in for bit steps. Each 4-bit value
+;is read, and its value is used as a bit pointer to the 'hashing'-register.
+;If a number is found, the bit in the hashing register is set. If the bit
+;is already set, the number has already been found the in original set.
+;To make the walk through easy, we use a special feature of the PDP-10:
+;We access the data register, as if it was a memory address.
+;The following registers are used:
+;A: Accumulator used to store the values that are read from the data field.
+;B: Used to check if a bit is set, without destroying the contents of I.
+;C: Contains the data field itself: 36 bits, in 4-bit segments.
+;E: Used as a table to store if a number was found in the field.
+;X: The byte pointer to the data field.
+CHKMAT: PUSH  P,A                      ;Save the register A
+        PUSH  P,C                      ;Save the data register
+        PUSH  P,E                      ;Save the E register
+        PUSH  P,X                      ;Save the index register
+        SET   E                        ;Set the hash register to 0.
+        MOVE  X,[440400,,C]            ;Store a byte pointer to C
+LOOP:   ILDB  A,X                      ;Store a byte in A
+        MOVEI B,E                      ;Copy the table.
+        LSH   B,A                      ;Shift out the bit to check.
+        ANDI  B,1                      ;Check if the first bit is set.
+