zeal8bit: fix a regression that happened after the keyboard refactoring

The interrupt handler needs to remap the kernel RAM to have access to the keyboard fifo.
This commit also fixes some comments.

target/zeal8bit/keyboard.asm

@@ -512,7 +512,7 @@ keyboard_fifo_size:
         ; Parameters:
         ;       A - Value to enqueue in the FIFO
         ; Returns:
-        ;       A - Non-zero value on success, 0 if FIFO empty
+        ;       -
         ; Alters:
         ;       A, HL
         PUBLIC keyboard_enqueue
@@ -601,8 +601,6 @@ kb_buffer_size: DEFS 1
         ; Index of the cursor in the internal buffer
         ; 0 <= cursor <= KB_INTERNAL_BUFFER_SIZE - 1
 kb_buffer_cursor: DEFS 1
-        ; State that will be updated according to the keys received in non-blocking mode
-kb_next_step: DEFS 2
 
         SECTION DRIVER_BSS_ALIGN16
         ALIGN 16

target/zeal8bit/keyboard/ps2.asm

@@ -55,7 +55,7 @@ keyboard_impl_next_key:
     call keyboard_dequeue
     ; If no character, return
     ret z
-    ; We just got a character, in D, check if we have to resume a previous
+    ; We just got a character, in A, check if we have to resume a previous
     ; step on hold (non-blocking mode)
     ld b, a
     ld hl, (kb_next_step)
@@ -235,9 +235,7 @@ extended_scan:
 
     PUBLIC keyboard_ps2_int_handler
 keyboard_ps2_int_handler:
-    ; Kernel RAM is now available!
-    ; In the keyboard interrupt handler, we will retrieve the key that has just been
-    ; pressed and put it in our FIFO
+    ; Enqueue the PS/2 scan code we just received
     in a, (KB_IO_ADDRESS)
     jp keyboard_enqueue
 

target/zeal8bit/pio.asm

@@ -3,6 +3,7 @@
 ; SPDX-License-Identifier: Apache-2.0
 
         INCLUDE "osconfig.asm"
+        INCLUDE "mmu_h.asm"
         INCLUDE "errors_h.asm"
         INCLUDE "drivers_h.asm"
         INCLUDE "pio_h.asm"
@@ -59,6 +60,12 @@ interrupt_default_handler:
 interrupt_pio_handler:
         ex af, af'
         exx
+        ; The kernel RAM may NOT BE MAPPED, we have to map it here
+        MMU_GET_PAGE_NUMBER(MMU_PAGE_3)
+        ; Save former page in D, we need it to restore it
+        ld d, a
+        MMU_MAP_KERNEL_RAM(MMU_PAGE_3)
+
         ; Check which pin triggered the interrupt, multiple pins can trigger
         ; this interrupt, so all pins shall be checked.
         in a, (IO_PIO_SYSTEM_DATA)
@@ -76,6 +83,10 @@ interrupt_pio_handler:
         bit IO_KEYBOARD_PIN, a
         call z, keyboard_ps2_int_handler
     ENDIF
+
+        ld a, d
+        MMU_SET_PAGE_NUMBER(MMU_PAGE_3)
+
         exx
         ex af, af'
         ei

target/zeal8bit/video.asm

@@ -510,13 +510,13 @@ stdout_print_buffer:
 ENDIF ; CONFIG_TARGET_STDOUT_VIDEO
 
     ; Routine called everytime a V-blank interrupt occurs
-    ; Must not alter A
+    ; Must not alter A, nor DE
     PUBLIC video_vblank_isr
 video_vblank_isr:
     ; Add 16(ms) to the counter
     ld hl, (vblank_count)
-    ld de, 16
-    add hl, de
+    ld bc, 16
+    add hl, bc
     ld (vblank_count), hl
     ret