lowlevel-z80: various (important) fixes

1.	Exit with reti, most platforms don't care but a few do so avoid
	digging holes

2.	MSX2 is the first platform we handle where not servicing an IRQ
	jams you in a loop. Provide a hook to always service some interrupts
	even if we are on an IRQ stack or otherwise detained from calling
	platform_interrupt

3.	The di() call saved the flags but forgot to DI, this fixes several
	rather odd bugs I'm sure

4.	The signal processing on irq return is suspect, note it for future
	sorting out.

With this and the MSX2 fixes we get closer to a basic MSX2 boot

diff --git a/Kernel/lowlevel-z80.s b/Kernel/lowlevel-z80.s
index 242e29f..558fbf2 100644 (file)
--- a/Kernel/lowlevel-z80.s
+++ b/Kernel/lowlevel-z80.s
@@ -27,6 +27,7 @@
        .globl _kernel_flag
        .globl _inint
        .globl _platform_interrupt
+       .globl platform_interrupt_all
 
         .module syscall
 
@@ -272,6 +273,11 @@ interrupt_handler:
             ld hl, (_system_tick_counter)
             inc hl
             ld (_system_tick_counter), hl
+
+           ; Some platforms (MSX for example) have devices we *must*
+           ; service irrespective of kernel state in order to shut them
+           ; up. This code must be in common and use small amounts of stack
+           call platform_interrupt_all
            ; FIXME: add profil support here (need to keep profil ptrs
            ; unbanked if so ?)
 
@@ -298,8 +304,8 @@ interrupt_handler:
            ; need to map anyway for trap_signal
            ;
            ld a, (_kernel_flag)
-           push af
            or a
+           push af
            jr nz, in_kernel
 
             ; we're not in kernel mode, check for signals and fault
@@ -349,6 +355,7 @@ int_switch:
             or a
             jr nz, interrupt_return
 
+           ; FIXME: check kernel mode flag ?
             ; we're not in kernel mode, check for signals
             call dispatch_process_signal
            ; FIXME: we should loop for multiple signals probably
@@ -369,7 +376,7 @@ interrupt_return:
             pop af
             ex af, af'
             ei
-            ret
+            reti
 
 ;  Enter with HL being the signal to send ourself
 trap_signal:
@@ -432,6 +439,7 @@ nmi_handler:
 _di:           ld a, i
                push af
                pop hl
+               di
                ret
 
 _irqrestore:   pop hl          ; sdcc needs to get register arg passing