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Added comment based on mailing list posting by Greg King.

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git-svn-id: svn://svn.cc65.org/cc65/trunk@4593 b7a2c559-68d2-44c3-8de9-860c34a00d81
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ol.sc 2010-02-20 09:52:05 +00:00
parent 2bd1f2f69f
commit 9ef9396b71
2 changed files with 36 additions and 0 deletions
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18
libsrc/c64/c64-1351.s
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@ -4,6 +4,24 @@
;
; 2009-09-26, Ullrich von Bassewitz
; 2010-02-06, Greg King
;
; The driver prevents the keyboard from interfering by changing the
; keyboard's output port into an input port while the driver reads its
; controller device. That disables a wire that is left active by the
; Kernal. That wire is used by the STOP-key to break out of BASIC
; programs -- CC65 programs don't use that feature. The wire is shared
; by these keys: STOP, "Q", Commodore, Space, "2", CTRL, Left-Arrow, and
; "1". I listed them, in order, from bit 7 over to bit 0. The
; rightmost five keys can look like joystick switches.
;
; The driver prevents the mouse/joystick from interfering by "blinding"
; the keyboard scanner while any button/switch is active. It changes
; the input port into an output port, then stores all zero-bits in that
; port's latch. Reading from an output port sees the bitwise-AND of the
; latch and the input signals. Therefore, the scanner thinks that eight
; keys are being pushed at the same time. It doesn't know what to do
; about that condition; so, it does nothing. The driver lets the
; scanner see normally, again, when no buttons/switches are active.
;
.include "zeropage.inc"

18
libsrc/c64/c64-joymouse.s
View File

@ -3,6 +3,24 @@
;
; Ullrich von Bassewitz, 2004-03-29, 2009-09-26
; 2010-02-08, Greg King
;
; The driver prevents the keyboard from interfering by changing the
; keyboard's output port into an input port while the driver reads its
; controller device. That disables a wire that is left active by the
; Kernal. That wire is used by the STOP-key to break out of BASIC
; programs -- CC65 programs don't use that feature. The wire is shared
; by these keys: STOP, "Q", Commodore, Space, "2", CTRL, Left-Arrow, and
; "1". I listed them, in order, from bit 7 over to bit 0. The
; rightmost five keys can look like joystick switches.
;
; The driver prevents the mouse/joystick from interfering by "blinding"
; the keyboard scanner while any button/switch is active. It changes
; the input port into an output port, then stores all zero-bits in that
; port's latch. Reading from an output port sees the bitwise-AND of the
; latch and the input signals. Therefore, the scanner thinks that eight
; keys are being pushed at the same time. It doesn't know what to do
; about that condition; so, it does nothing. The driver lets the
; scanner see normally, again, when no buttons/switches are active.
;
.include "zeropage.inc"