feat(kernal): $E5B4, $EA24, and $ECF0 at their well-known addresses

cc65's C64 runtime and ip65 call past the jump table: $E5B4 takes the first
key from the keyboard buffer (cgetc), $EA24 points USER at the colour RAM of
PNT's line (cursor positioning), and $ECF0 holds the low bytes of the screen
lines' addresses (ip65's VT100 code), as Mapping the Commodore 64 describes
them. vicii_init's work moves out of $E5A0-$E5C8 to free $E5B4, and on the
standard layout PNT and USER are worked out through the routine at $EA24.

src/kernal/init/e5a0.vicii_init.s

@@ -22,43 +22,9 @@ vicii_init:
 
 } else {
 
-	; Clear everything - turn off sprites (observed hanging around after
- 	; running programs and resetting), etc.
+	; The work is elsewhere: $E5B4, inside the 41 bytes it takes, is the
+	; keyboard buffer's well-known entry (e5b4.get_from_keyboard_buffer.s)
 
-	lda #$00
- 	ldx #$2E
-@1:
- 	sta __VIC_BASE, x
- 	dex
- 	bpl @1
-
-	; Set up default IO values
-	; - [CM64] page 129       - VIC_SCROLY
-	; - [CM64] pages 140-144  - VIC_SCROLX
-	; - [CM64] pages 145-146  - VIC_YMCSB
-
-	lda #$1B                           ; according to [CM64] page 137 some software assumes oldest bit is 0
-	sta VIC_SCROLY
-	lda #$C8
-	sta VIC_SCROLX                     ; 40 column, etc.
-	lda #$14
-	sta VIC_YMCSB
-	lda #$0F
-	sta VIC_IRQ                        ; clear all interrupts
-
-	; We use a different colour scheme of white text on all blue by default,
-	; but brand-specific color scheme can also be enabled
-
-	lda #CONFIG_COLOR_BG
-	sta VIC_EXTCOL
-	sta VIC_BGCOL0
-
-	; Setup default I/O devices
-
-!ifdef CONFIG_IEC {
-	jmp clrchn_reset
-} else {
-	jmp CLRCHN
-}
+	jmp vicii_init_registers
 
 } ; ROM layout

src/kernal/init/vicii_init_registers.s

@@ -0,0 +1,52 @@
+;; #LAYOUT# STD *        #TAKE
+;; #LAYOUT# CRT KERNAL_0 #TAKE
+;; #LAYOUT# M65 KERNAL_1 #TAKE
+;; #LAYOUT# *   *        #IGNORE
+
+;
+; The VIC-II set-up of vicii_init (e5a0.vicii_init.s), moved out of $E5A0-$E5C8
+; so that $E5B4 is free
+;
+
+
+vicii_init_registers:
+
+	; Clear everything - turn off sprites (observed hanging around after
+ 	; running programs and resetting), etc.
+
+	lda #$00
+ 	ldx #$2E
+@1:
+ 	sta __VIC_BASE, x
+ 	dex
+ 	bpl @1
+
+	; Set up default IO values
+	; - [CM64] page 129       - VIC_SCROLY
+	; - [CM64] pages 140-144  - VIC_SCROLX
+	; - [CM64] pages 145-146  - VIC_YMCSB
+
+	lda #$1B                           ; according to [CM64] page 137 some software assumes oldest bit is 0
+	sta VIC_SCROLY
+	lda #$C8
+	sta VIC_SCROLX                     ; 40 column, etc.
+	lda #$14
+	sta VIC_YMCSB
+	lda #$0F
+	sta VIC_IRQ                        ; clear all interrupts
+
+	; We use a different colour scheme of white text on all blue by default,
+	; but brand-specific color scheme can also be enabled
+
+	lda #CONFIG_COLOR_BG
+	sta VIC_EXTCOL
+	sta VIC_BGCOL0
+
+	; Setup default I/O devices
+
+!ifdef CONFIG_IEC {
+	jmp clrchn_reset
+} else {
+	jmp CLRCHN
+}
+

src/kernal/keyboard/e5b4.get_from_keyboard_buffer.s

@@ -0,0 +1,21 @@
+;; #LAYOUT# STD *        #TAKE
+;; #LAYOUT# *   *        #IGNORE
+
+;
+; Well-known Kernal routine, described in:
+;
+; - [CM64] Computes Mapping the Commodore 64 - $E5B4, LP2
+;
+; Takes the first key from the keyboard buffer, which must not be empty, and moves the
+; others up. Out: the key in A. X is kept.
+;
+
+
+get_from_keyboard_buffer:
+
+	lda KEYD
+	pha
+	jsr pop_keyboard_buffer
+	pla
+
+	rts

src/kernal/keyboard/e5b4.interop

@@ -0,0 +1,3 @@
+cc65's C64 runtime calls $E5B4 (KBDREAD in its cbm_kernal.inc) from cgetc, once the keyboard
+buffer holds a key, and takes the key from A; so does every cc65 program that reads a key,
+Contiki's among them.

src/kernal/screen/ea24.interop

@@ -0,0 +1,2 @@
+cc65's C64 runtime calls $EA24 (UPDCRAMPTR in its cbm_kernal.inc) after PLOT has set the cursor,
+for USER to follow PNT, before it stores a character's colour through USER.

src/kernal/screen/ea24.screen_calculate_user.s

@@ -0,0 +1,24 @@
+;; #LAYOUT# STD *        #TAKE
+;; #LAYOUT# *   *        #IGNORE
+
+;
+; Well-known Kernal routine, described in:
+;
+; - [CM64] Computes Mapping the Commodore 64 - $EA24
+;
+; Sets USER to the colour RAM location of PNT's screen line. Screen RAM starts on a
+; kilobyte boundary, as the VIC-II fetches it, so the line's place within its kilobyte
+; is its place in the colour RAM.
+;
+
+
+screen_calculate_USER:
+
+	lda PNT+0
+	sta USER+0
+	lda PNT+1
+	and #$03
+	ora #>COLOR_RAM
+	sta USER+1
+
+	rts

src/kernal/screen/ecf0.interop

@@ -0,0 +1,2 @@
+ip65's VT100 emulation (drivers/c64vt100.s) reads the low byte of a screen line's address from
+$ECF0, indexed by the line number.

src/kernal/screen/ecf0.screen_line_lo_bytes.s

@@ -0,0 +1,17 @@
+;; #LAYOUT# STD *        #TAKE
+;; #LAYOUT# *   *        #IGNORE
+
+;
+; Well-known Kernal table, described in:
+;
+; - [CM64] Computes Mapping the Commodore 64 - $ECF0
+;
+; The low bytes of the 25 screen lines' addresses, 40 bytes apart
+;
+
+
+screen_line_lo_bytes:
+
+	!for line, 0, 24 {
+		!byte <(line * 40)
+	}

src/kernal/screen/screen_calculate_pnt_user.s

@@ -26,7 +26,7 @@ screen_calculate_PNT_USER_from_X: ; entry point for screen_clear_line
 	; Add 40 to PNT for every line
 
 	cpx #$00	
-	beq screen_calculate_USER
+	beq @3
 @1:
 	lda #40
 	clc
@@ -37,8 +37,13 @@ screen_calculate_PNT_USER_from_X: ; entry point for screen_clear_line
 @2:
 	dex
 	bne @1
+@3:
 
-	; FALLTROUGH
+!ifdef ROM_LAYOUT_STD {
+
+	jmp screen_calculate_USER          ; at its well-known address, $EA24
+
+} else {
 
 screen_calculate_USER:
 
@@ -54,3 +59,4 @@ screen_calculate_USER:
 	sta USER+1
 
 	rts
+}