From 72dfa39b81269bb824a43b464a0ea8676bd5e631 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 23 Oct 2022 13:03:25 -0700
Subject: Add untested interrupt-based echo program with input buffer

---
 programs/echo.pal | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 programs/echo.pal

(limited to 'programs')

diff --git a/programs/echo.pal b/programs/echo.pal
new file mode 100644
index 0000000..16d8fda
--- /dev/null
+++ b/programs/echo.pal
@@ -0,0 +1,67 @@
+/ vim: set sw=8 noexpandtab :
+
+*000
+
+INTRET,	0
+	JMP GOTINT
+
+INPUTY,	1000	/ Address of youngest byte in input buffer
+INPUTO,	1000	/ Address of oldest byte in input buffer
+
+*200
+
+/ Main program enables keyboard interrupt, then enters echo loop
+	KIE
+	ION
+LOOP,	TSF		/ Call ECHO if printer is ready
+	JMS ECHO
+	JMP LOOP
+
+ECHO,	0
+	TAD INPUTO
+	CMA IAC		/ Negate
+	TAD INPUTY
+	SNA		/ Abandon echo if input buffer is empty (n.b. or full, because we don't have wrap bits)
+	JMP I ECHO
+	TAD I INPUTO	/ Read from input buffer
+	TLS		/ Print
+	JMS FIXO
+	JMP I ECHO
+
+GOTINT,	KSF		/ Skip reading keyboard if nothing was typed
+	JMP NOIN
+	DCA SAVED
+	KRB		/ Read from keyboard
+	DCA I INPUTY	/ Write to input buffer
+	JMS FIXY
+	CLA
+	TAD SAVED
+NOIN,	RMF
+	ION
+	JMP I INTRET
+
+/ Wrap INPUTY pointer at 2000 back to 1000
+FIXY,	0
+	TAD INPUTY
+	TAD [-2000]
+	SZA
+	JMP I FIXY
+	CLA
+	TAD [1000]
+	DCA INPUTY
+	JMP I FIXY
+
+/ Wrap INPUTO pointer at 2000 back to 1000
+FIXO,	0
+	TAD INPUTO
+	TAD [-2000]
+	SZA
+	JMP I FIXO
+	CLA
+	TAD [1000]
+	DCA INPUTO
+	JMP I FIXO
+
+*300
+
+SAVED,	0
-- 
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