Banking

boot.asm

@@ -2,6 +2,12 @@ STACK_TOP = 0xFFFF
 DELAY_LOOPS_PER_MS = 40
 RESIDENT_BASE = 0xC000
 RESIDENT_ROM_SOURCE = 0x0080
+BANK_CONTROL_PORT = 0x00
+BOOT_BANK_MAPPING = 0x06   ; ROM on, banking on, upper 16K at C000-FFFF
+ROM_DISABLE_BIT = 0x01
+BANK_SELECT_BIT = 0x02
+BANK_ENABLE_BIT = 0x04
+BANK_MODE_CLEAR_MASK = 0xF9 ; preserve ROM-disable and auxiliary outputs
 
         .include "bios.inc"
 
@@ -16,6 +22,9 @@ RESIDENT_ROM_SOURCE = 0x0080
         .globl _bios_try_getc
         .globl _bios_getc
         .globl _bios_init
+        .globl _bios_bank
+        .globl _bios_rom_disable
+        .globl _bank_control_shadow
         .globl resident_image_end
         .globl s__DATA
         .globl l__DATA
@@ -30,6 +39,10 @@ RESIDENT_ROM_SOURCE = 0x0080
 .org 0x08
 boot_copy:
         di
+        ; Keep ROM enabled while mapping the upper physical 16K at C000-FFFF.
+        ; The resident copy therefore survives either banked mode after boot.
+        ld a, #BOOT_BANK_MAPPING
+        out (BANK_CONTROL_PORT), a
         ld sp, #STACK_TOP
         ld hl, #RESIDENT_ROM_SOURCE
         ld de, #RESIDENT_BASE
@@ -66,7 +79,8 @@ boot_copy:
 
 ; Stable BIOS jump table for ROM-resident applications.
 ; C050 putc(A), C053 try_getc(), C056 getc(), C059 io_out(A,L),
-; C05C io_in(A), C05F delay_ms(HL), C062 monitor warm start.
+; C05C io_in(A), C05F delay_ms(HL), C062 monitor warm start,
+; C065 bank(A), C068 disable ROM.
 .org BIOS_PUTC
         jp bios_service_putc
 .org BIOS_TRY_GETC
@@ -81,6 +95,17 @@ boot_copy:
         jp bios_service_delay_ms
 .org BIOS_MONITOR
         jp init
+.org BIOS_BANK
+        jp _bios_bank
+.org BIOS_ROM_DISABLE
+        jp _bios_rom_disable
+
+; Port 00 is write-only.  Keep its last BIOS-managed value outside the C data
+; areas so a monitor warm start does not reset it.  The bootstrap establishes
+; this same value in the hardware before the resident image is copied.
+.org RESIDENT_BASE + 0x70
+_bank_control_shadow::
+        .db BOOT_BANK_MAPPING
 
 .area _HOME
 init:
@@ -135,6 +160,41 @@ resident_image_end:
 
 .area _CODE
 
+; Select a RAM mapping while retaining the shadowed ROM-disable bit.
+; Input: A=0 selects lower 48K + upper 16K; any other value selects upper 64K.
+; Both modes keep banking enabled.  A and flags are preserved.
+_bios_bank::
+        push    af
+        or      a
+        jr      z, bios_bank_split
+
+        ld      a, (_bank_control_shadow)
+        and     #BANK_MODE_CLEAR_MASK
+        or      #BANK_ENABLE_BIT
+        jr      bios_bank_update
+
+bios_bank_split:
+        ld      a, (_bank_control_shadow)
+        and     #BANK_MODE_CLEAR_MASK
+        or      #(BANK_ENABLE_BIT | BANK_SELECT_BIT)
+
+bios_bank_update:
+        ld      (_bank_control_shadow), a
+        out     (BANK_CONTROL_PORT), a
+        pop     af
+        ret
+
+; Permanently hide ROM until reset, preserving the selected RAM mapping.
+; A and flags are preserved.
+_bios_rom_disable::
+        push    af
+        ld      a, (_bank_control_shadow)
+        or      #ROM_DISABLE_BIT
+        ld      (_bank_control_shadow), a
+        out     (BANK_CONTROL_PORT), a
+        pop     af
+        ret
+
 ; BIOS ABI services preserve registers except documented return values.
 bios_service_putc:
         push    af