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Add BRK breakpoint dispatch to the break monitor #40

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2310cba
feat(kernel): dispatch emulation-mode BRK into break monitor
mbrukner Aug 22, 2026
e8817d1
fix(kernel): align JR2 break monitor integration
mbrukner Sep 1, 2026
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188 changes: 38 additions & 150 deletions f256/jr.asm
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Original file line number Diff line number Diff line change
Expand Up @@ -88,14 +88,14 @@ nmi_saved_mmu .byte ? ; victim mmu_ctrl (active LUT) at NMI
nmi_saved_io .byte ? ; victim io_ctrl at NMI
nmi_saved_slot5 .byte ? ; (unused since Task1) legacy LUT0 slot-5 save
nmi_in_progress .byte ? ; non-zero while a handler is active (nested guard)
nmi_entry_src .byte ? ; 0 = Foenix-key break, 1 = BRK (breakpoint/step)
nmi_ptr .word ? ; (unused since Task1) legacy header entry pointer
nmi_orig_slot4 .byte ? ; victim LUT slot-4 bank displaced by the handler
nmi_orig_slot5 .byte ? ; victim LUT slot-5 bank displaced by the handler
nmi_save2 .byte ? ; LUT0 slot-2 saved during the flash->RAM copy
nmi_save4 .byte ? ; LUT0 slot-4 saved during the flash->RAM copy
nmi_src .word ? ; flash->RAM copy source pointer
nmi_dst .word ? ; flash->RAM copy dest pointer
nmi_fillb .byte ? ; fill byte for screen clear
.send

.section startup ; The following is ALWAYS at $E000
Expand Down Expand Up @@ -288,11 +288,9 @@ VIA_IER = $DC0E ; 65C22 VIA Interrupt Enable Register
NMI_MON_BANK = $4a ; monitor flash block 0 ($40 + CSV 0a); block 1 = $4b
NMI_RAM_LO = $3e ; reserved RAM bank -> victim slot 4 ($8000): image block 0
NMI_RAM_HI = $3f ; reserved RAM bank -> victim slot 5 ($A000): image block 1
NMI_TEXT_BANK = $3c ; reserved RAM bank: saved victim text page
NMI_COLR_BANK = $3d ; reserved RAM bank: saved victim color page
NMI_SCR_CLEAR = $10 ; cleared color attribute (FG=white, BG=black)
NMI_MEMTEXT_LO = $3b ; reserved RAM banks $3b-$3d: monitor MemText/state
MACHINE_ID_REG = $d6a7
JR2_MID = $18
JR2_MID = $22 ; $A2 on current JR2 cores, ignoring clock bits [7:6]
JR2_SYSRQ_CTRL = $d6b6 ; bit 7: sticky SysRq NMI cause, W1C

hw_nmi:
Expand All @@ -317,7 +315,7 @@ hw_nmi:
; SysRq NMI cause in $D6B6 bit 7. Accept and acknowledge only that cause;
; an IEC NMI must not enter the monitor accidentally.
lda MACHINE_ID_REG
and #$1f
and #$3f
cmp #JR2_MID
bne _nmi_gate_k2
lda JR2_SYSRQ_CTRL
Expand All @@ -333,10 +331,13 @@ _nmi_gate_k2
beq _nmi_ours
jmp nmi_not_ours ; Foenix not held -> ignore
_nmi_ours
stz nmi_entry_src ; 0 = entered via Foenix key
; Decline a break INTO the monitor itself: it runs from reserved bank
; NMI_RAM_HI ($3f) in slot 5, so freezing it would clobber its own RAM
; during the flash->RAM copy and could not be resumed. If the victim's
; slot 5 is that bank, ignore the NMI and resume it untouched.
; (The BRK path in hw_irq joins here, having set nmi_entry_src = 1.)
nmi_selfguard:
jsr nmi_edit_vlut
lda mmu+5
stz mmu_ctrl ; active LUT0, edit off
Expand All @@ -349,9 +350,6 @@ _nmi_notmon
lda io_ctrl
sta nmi_saved_io

; c2: save the victim's text+color screen and clear it for the monitor
jsr nmi_save_screen

; --- save the victim's original slot-4 and slot-5 banks ---
jsr nmi_edit_vlut ; edit-enable the victim's LUT
lda mmu+4
Expand Down Expand Up @@ -380,7 +378,16 @@ _nmi_notmon
jsr nmi_copy8k
; poke the victim's slot-4 bank into the RAM image's reserved header byte
; (offset 9); RAM_LO is mapped at LUT0 slot4 ($8000), so that is $8009.
; Bank numbers are <= $3f, so bit 7 is free: set it when the entry was a
; BRK (breakpoint/step) so the monitor can tell BRK from a Foenix break.
lda nmi_entry_src
beq _poke_key
lda nmi_orig_slot4
ora #$80
bra _poke_do
_poke_key
lda nmi_orig_slot4
_poke_do
sta $8009
; block 1: flash NMI_MON_BANK+1 -> RAM_HI
lda #$80
Expand Down Expand Up @@ -424,9 +431,6 @@ _nmi_notmon
sta mmu+5
stz mmu_ctrl

; c2: restore the victim's text+color screen
jsr nmi_restore_screen

; --- inject a Foenix-key RELEASE for the resumed program ---
; The victim cached the Foenix PRESS before the break but never saw the
; release (the monitor consumed it), so it still thinks Foenix is held.
Expand Down Expand Up @@ -512,142 +516,6 @@ _nc1 lda (nmi_src),y
bne _nc
rts

; ---- Screen save / restore (c2) ---------------------------------------
; Runs under LUT0. Saves the victim's text + color pages into reserved
; RAM banks and clears the screen for the monitor; restores on exit.
; The 80x60 text/color screens are 4800 bytes each at $C000 (io page 2
; = text, io page 3 = color); a reserved bank is mapped into LUT0 slot 4
; ($8000) as the copy buffer.

; nmi_cp4800: copy 4800 bytes (nmi_src)->(nmi_dst).
nmi_cp4800
ldx #18 ; 18 full pages
_cp0 ldy #0
_cp1 lda (nmi_src),y
sta (nmi_dst),y
iny
bne _cp1
inc nmi_src+1
inc nmi_dst+1
dex
bne _cp0
ldy #0 ; + 192 bytes = 4800
_cp2 lda (nmi_src),y
sta (nmi_dst),y
iny
cpy #192
bne _cp2
rts

; nmi_fill4800: fill 4800 bytes at $C000 with nmi_fillb (current io page).
nmi_fill4800
stz nmi_dst
lda #$c0
sta nmi_dst+1
ldx #18
_fl0 ldy #0
_fl1 lda nmi_fillb
sta (nmi_dst),y
iny
bne _fl1
inc nmi_dst+1
dex
bne _fl0
ldy #0
_fl2 lda nmi_fillb
sta (nmi_dst),y
iny
cpy #192
bne _fl2
rts

; map bank A into LUT0 slot 4 ($8000), saving the displaced bank
nmi_map4
pha
lda #$80
sta mmu_ctrl ; edit LUT0
lda mmu+4
sta nmi_save4
pla
sta mmu+4
stz mmu_ctrl
rts
nmi_unmap4
lda #$80
sta mmu_ctrl
lda nmi_save4
sta mmu+4
stz mmu_ctrl
rts

; nmi_save_screen: save text->$3d, color->$3e, clear screen + home cursor
nmi_save_screen
lda #NMI_TEXT_BANK
jsr nmi_map4 ; $3d -> slot 4 ($8000)
lda #2
sta io_ctrl ; text page at $C000
stz nmi_src
lda #$c0
sta nmi_src+1 ; src = $C000
stz nmi_dst
lda #$80
sta nmi_dst+1 ; dst = $8000
jsr nmi_cp4800
lda #NMI_COLR_BANK
jsr nmi_map4 ; $3e -> slot 4 (nmi_save4 reused; ok, sequential)
lda #3
sta io_ctrl ; color page at $C000
stz nmi_src
lda #$c0
sta nmi_src+1
stz nmi_dst
lda #$80
sta nmi_dst+1
jsr nmi_cp4800
jsr nmi_unmap4
; clear text (spaces) + color (attr), home the hardware cursor
lda #2
sta io_ctrl
lda #$20
sta nmi_fillb
jsr nmi_fill4800
lda #3
sta io_ctrl
lda #NMI_SCR_CLEAR
sta nmi_fillb
jsr nmi_fill4800
lda #2
sta io_ctrl ; leave text page selected
rts ; cursor is homed by the monitor (after it saves
; the victim's cursor regs in its c1 block)

; nmi_restore_screen: restore text from $3d, color from $3e
nmi_restore_screen
lda #NMI_TEXT_BANK
jsr nmi_map4
lda #2
sta io_ctrl
stz nmi_src
lda #$80
sta nmi_src+1 ; src = $8000 ($3d)
stz nmi_dst
lda #$c0
sta nmi_dst+1 ; dst = $C000 (text)
jsr nmi_cp4800
lda #NMI_COLR_BANK
jsr nmi_map4
lda #3
sta io_ctrl
stz nmi_src
lda #$80
sta nmi_src+1
stz nmi_dst
lda #$c0
sta nmi_dst+1
jsr nmi_cp4800
jsr nmi_unmap4
rts

; Not our modifier combo: save area is valid (we are the outer handler);
; restore victim LUT/SP/regs without any slot changes.
nmi_not_ours
Expand All @@ -668,7 +536,7 @@ nmi_reenter
; mistaken for that old key press. Preserve A, which holds the victim LUT.
pha
lda MACHINE_ID_REG
and #$1f
and #$3f
cmp #JR2_MID
bne _nmi_reenter_restore
lda JR2_SYSRQ_CTRL
Expand Down Expand Up @@ -697,6 +565,27 @@ hw_irq:
phx
phy

; Distinguish a BRK ($00) from a hardware IRQ: in emulation mode the CPU
; pushes the status with bit 4 (B) = 1 for BRK, 0 for IRQ. A,X,Y were just
; pushed above the interrupt frame, so pushed-P is at SP+4.
tsx
lda $0104,x
and #$10
beq _hw_irq_norm
; --- BRK: dispatch into the break monitor (mirror the hw_nmi prologue,
; but skip the Foenix gate; X = victim SP from the tsx above). ---
lda mmu_ctrl ; A = victim active LUT
stz mmu_ctrl ; -> kernel LUT (LUT0)
ldy nmi_in_progress
beq +
jmp nmi_reenter
+ sta nmi_saved_mmu
stx nmi_saved_sp
lda #1
sta nmi_entry_src ; 1 = entered via BRK
jmp nmi_selfguard

_hw_irq_norm
; Save MMU state and switch to the kernel's MMU table.
lda mmu_ctrl ; Get the current mmu state.
stz mmu_ctrl ; Switch to the kernel's mmu table.
Expand Down Expand Up @@ -800,4 +689,3 @@ k2_feature_detect

.send
.endn