Interrupt brings up Monitor: working, but calling "run" leaves garbage in the SP

pat80-computer/software/z80-assembly/os/abi-generated.asm

@@ -1,6 +1,6 @@
-Sys_ABI:	equ $0003
-Sys_Beep:	equ $0013
-Sys_Print:	equ $0007
-Sys_Printc:	equ $000a
-Sys_Readc:	equ $000d
-Sys_Readline:	equ $0010
+Sys_ABI:	equ $0081
+Sys_Beep:	equ $0091
+Sys_Print:	equ $0085
+Sys_Printc:	equ $0088
+Sys_Readc:	equ $008b
+Sys_Readline:	equ $008e

pat80-computer/software/z80-assembly/os/main.asm

@@ -19,9 +19,21 @@ jp Sysinit     ; Startup vector: DO NOT MOVE! Must be the first instruction
 ;   I/O 6 (0xC0 - 0xDF)
 ;   I/O 7 (0xE0 - 0xFF)
 
+; **** RESET/INTERRUPT VECTOR ****
+
+; Maskable interrupt mode 1: when the BREAK key is pressed, 
+; a maskable interrupt is generated and the CPU jumps to this address.
+; In this way, BREAK key brings up memory monitor at any time.
+ds 0x38
+	di ; Disable maskable interrupts.
+	exx ; exchange registers
+	ex af, af'
+	jp Monitor_main
+
 ; **** SYSTEM CALLS ****
 ; System calls provide access to low level functions (input from keyboard, output to screen etc).
 ; The name starts always with Sys_
+ds 0x40	; Place system calls after Z80 reset/interrupt subroutines space
 
 ; Returns ABI version.
 ; (ABI -> https://en.wikipedia.org/wiki/Application_binary_interface)
@@ -101,13 +113,18 @@ Sysinit:
     ; Play startup sound
     call Sys_Beep
 
-    ; Run memory monitor
-    call Monitor_main
+	; Run memory monitor
+	ei ; enable maskabpe interrupts
+	im 1 ; set interrupt mode 1 (on interrupt jumps to 0x38)
+	rst 0x38 ; throw fake interrupt: jump to interrupt routine to start monitor
+
+	; User exited from memory monitor without loading a program. Do nothing.
+	mloop:
+		; Main loop: do nothing.
+		jp mloop
 
     ; DEBUG: Echo chars
     ; loop:
     ;     call Term_readc
     ;     call Term_printc
     ;     jp loop
-
-    halt

pat80-computer/software/z80-assembly/os/monitor.asm

@@ -21,8 +21,9 @@ MON_COMMAND_ZERO: DB "ZERO",0
 MON_COMMAND_LOAD: DB "LOAD",0
 MON_COMMAND_RUN: DB "RUN",0
 MON_COMMAND_ADB: DB "ADB",0
+MON_COMMAND_QUIT: DB "QUIT",0
 MON_ARG_HEX: DB "    0x",0
-MON_HELP: DB 10,"Available commands:\nHELP prints this message\nDUMP [ADDR] shows memory content\nSET [ADDR] sets memory content\nZERO [ADDR] [ADDR] sets all bytes to 0 in the specified range\nLOAD\nRUN [ADDR] executes code starting from ADDR\nADB starts Assembly Deploy Bridge",0
+MON_HELP: DB 10,"Available commands:\nHELP prints this message\nDUMP [ADDR] shows memory content\nSET [ADDR] sets memory content\nZERO [ADDR] [ADDR] sets all bytes to 0 in the specified range\nLOAD\nRUN [ADDR] executes code starting from ADDR\nADB starts Assembly Deploy Bridge\nQUIT exits",0
 MON_MSG_ADB: DB 10,"Waiting for data.",0
 MON_ERR_SYNTAX: DB "    Syntax error",0
 ;MON_ADB_TIMEOUT: EQU 0xFF     // Number of cycles after an ADB binary transfer is considered completed
@@ -66,6 +67,9 @@ Monitor_main:
         ld hl, MON_COMMAND_ADB
         cp (hl)
         jp z, monitor_adb
+        ld hl, MON_COMMAND_QUIT
+		cp (hl)
+		jp z, monitor_quit
         ; Unrecognized command: print error and beep
         ld bc, MON_ERR_SYNTAX
         call Sys_Print
@@ -80,6 +84,23 @@ monitor_help:
     call Sys_Print
     jp monitor_main_loop
 
+monitor_quit:
+    ld bc, MON_COMMAND_QUIT + 1 ; autocomplete command
+    call Sys_Print
+	; newline
+	ld a, 10
+	call Sys_Printc
+    ; Restores registers and re-enable interrupts: when the BREAK key is pressed, 
+	; a maskable interrupt is generated and the CPU jumps to 0x38 reset vector, 
+	; where if finds a call to Memory monitor (see main.asm).
+	exx ; exchange registers
+	ex af, af'
+	; enable interrupts
+	ei
+	im 1 ; set interrupt mode 1 (on interrupt jumps to 0x38)
+	reti ; return from interrupt
+
+
 ; Asks the user for a memory position and shows the following 64 bytes of memory
 ; @uses a, b, c, d, e, h, l
 monitor_dump:
@@ -257,23 +278,34 @@ monitor_zero: ; TODO: bugged, doesn't exit cycle
 monitor_load:
     ld bc, MON_COMMAND_LOAD + 1 ; autocomplete command
     call Sys_Print
+	; TODO: When implemented, re-enable interrupts before run application
     jp monitor_main_loop
 
 monitor_run:
     ld bc, MON_COMMAND_RUN + 1 ; autocomplete command
     call Sys_Print
-    ; Now read the memory address to be changed from the user
+    ; Now read the memory address to be executed from the user
     call monitor_arg_2byte  ; returns the read bytes in hl
     ld a, 10 ; newline
-    call Sys_Printc
-    jp (hl)    ; Start executing code
+    call Sys_Printc	
+	; enable interrupts
+	ei
+	im 1 ; set interrupt mode 1 (on interrupt jumps to 0x38)
+	; pop the last entry on the stack: this is needed (as the monitor
+	; runs in an interrupt) to counter-balance the missing reti statement
+	pop bc
+	; execute code
+	jp (hl)
 
 monitor_adb:
     ld bc, MON_COMMAND_ADB + 1 ; autocomplete command
     call Sys_Print
     ; start copying incoming data to application space
     call monitor_copyTermToAppMem
+	; call monitor_enable_int    ; re-enable interrupts
     ;jp APP_SPACE    ; Start executing code
+
+
     ; ld bc, APP_SPACE
     ; call Sys_Print
     jp monitor_main_loop
@@ -517,3 +549,5 @@ monitor_copyTermToAppMem:
         jp monitor_copyTermToAppMem_loop   ; continue loop
 
 
+
+