Add untested interrupt-based echo program with input buffer

1 files changed, 67 insertions, 0 deletions

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