snes.inc: add symbols for high-byte

In #3, a user of the template reported confusion between reading
the controller's entire report as a 16-bit unit and reading only
the high byte as an 8-bit unit.  To make this confusion less
likely going forward, add a set of labels and constants for
reading the high byte for convenience in porting NES code.

src/player.s

@@ -58,9 +58,11 @@ player_facing:    .res 1
   rtl
 .endproc
 
-; except for STZs and BRAs, the following subroutine is
-; direct copypasta from the NES template code
-cur_keys := JOY1CUR+1
+; Except for STZs and BRAs, the following subroutine is copied and
+; pasted from the NES template code.  We use JOY1CUR_HI for this
+; particular routine because that most directly corresponds to the
+; bit layout of the NES controller.  When using JOYxCUR_HI, we also
+; use the matching set of KEY_HI_* constants.
 
 ; constants used by move_player
 ; PAL frames are about 20% longer than NTSC frames.  So if you make
@@ -81,8 +83,8 @@ RIGHT_WALL = 224
   ; Acceleration to right: Do it only if the player is holding right
   ; on the Control Pad and has a nonnegative velocity.
   setaxy8
-  lda cur_keys
-  and #>KEY_RIGHT
+  lda JOY1CUR_HI
+  and #KEY_HI_RIGHT
   beq notRight
   lda player_dxlo
   bmi notRight
@@ -115,8 +117,8 @@ RIGHT_WALL = 224
 
   ; Acceleration to left: Do it only if the player is holding left
   ; on the Control Pad and has a nonpositive velocity.
-  lda cur_keys
-  and #>KEY_LEFT
+  lda JOY1CUR_HI
+  and #KEY_HI_LEFT
   beq notLeft
   lda player_dxlo
   beq isLeft

src/snes.inc

@@ -310,7 +310,7 @@ JOYOUT := $4201
 ; +-------- Controller 2 pin 6 output
 
 ; Results of the autoreader
-JOY1CUR   := $4218    ; Bit 0: used by standard controllers
+JOY1CUR   := $4218  ; Bit 0: used by standard controllers
 JOY2CUR   := $421A
 JOY1B1CUR := $421C  ; Bit 1: used by multitap and a few oddball
 JOY2B1CUR := $421E  ; input devices
@@ -334,6 +334,22 @@ KEY_X      = $0040
 KEY_L      = $0020
 KEY_R      = $0010
 
+; It is also possible to read the controller by reading only the
+; upper 8 bits in 8-bit mode (m=1).  This may be the path of least
+; resistance for porting NES software to the Super NES.  Be careful
+; not to mix _HI addresses with non-_HI constants or vice versa.
+; A, X, L, and R cannot be read this way.
+JOY1CUR_HI   := JOY1CUR + 1
+JOY2CUR_HI   := JOY2CUR + 1
+KEY_HI_B      = .hibyte(KEY_B)
+KEY_HI_Y      = .hibyte(KEY_Y)
+KEY_HI_SELECT = .hibyte(KEY_SELECT)
+KEY_HI_START  = .hibyte(KEY_START)
+KEY_HI_UP     = .hibyte(KEY_UP)
+KEY_HI_DOWN   = .hibyte(KEY_DOWN)
+KEY_HI_LEFT   = .hibyte(KEY_LEFT)
+KEY_HI_RIGHT  = .hibyte(KEY_RIGHT)
+
 ; S-CPU multiply and divide ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ; Multiply unit.  Also good for shifting pixels when drawing