From 7f909b4f75dbc8b6affe0af9c6e0be096447000f Mon Sep 17 00:00:00 2001
From: "Matthias@Dell" <matthiasqui@protonmail.com>
Date: Sun, 31 Dec 2023 14:56:24 +0100
Subject: [PATCH] add parity

---
 util/parity.h65 | 35 +++++++++++++++++++++++++++++++++++
 util/parity.s65 | 23 +++++++++++++++++++++++
 2 files changed, 58 insertions(+)
 create mode 100644 util/parity.h65
 create mode 100644 util/parity.s65

diff --git a/util/parity.h65 b/util/parity.h65
new file mode 100644
index 0000000..3b78ad3
--- /dev/null
+++ b/util/parity.h65
@@ -0,0 +1,35 @@
+.ifndef INCLUDE_PARITY
+INCLUDE_PARITY = 1
+
+.import count_set_bits
+
+;;********************************************************************************
+;; @macro Calculate even parity
+;; @modifies: A, X, Y
+;; @returns A: 0 - even number of set bits, 1 - uneven number of set bits
+;; @returns Y: The number of set bits in the byte
+;;********************************************************************************
+.macro CalculateEvenParity
+    jsr count_set_bits
+    tya
+    and #1
+.endmacro
+
+
+;;********************************************************************************
+;; @macro Calculate odd parity
+;; @details
+;;  Count the number of set bits in Y.
+;;  
+;; @modifies: A, X, Y
+;; @returns A: 1 - even number of set bits, 0 - uneven number of set bits
+;; @returns Y: The number of set bits in the byte
+;;********************************************************************************
+.macro CalculateOddParity
+    jsr count_set_bits
+    tya
+    and #1
+    eor #1
+.endmacro
+
+.endif  ; guard
diff --git a/util/parity.s65 b/util/parity.s65
new file mode 100644
index 0000000..3ef087c
--- /dev/null
+++ b/util/parity.s65
@@ -0,0 +1,23 @@
+.export count_set_bits
+;;********************************************************************************
+;; @function Initialize the PS2 keyboard
+;; @details
+;;  Count the number of set bits in Y.
+;;  
+;; @modifies: A, X, Y
+;; @returns Y: The number of set bits in the byte
+;;********************************************************************************
+.proc count_set_bits
+    ldx #8
+    ldy #0  ; number of set bits
+@loop:
+    dex 
+    beq @done
+    ror
+    bcc @loop
+@set:
+    iny
+    bra @loop
+@done:
+    rts
+.endproc