From 08b32b20641e3d4193618e9edc1e6868555e52ac Mon Sep 17 00:00:00 2001
From: "Daniele Verducci (ZenPenguin)" <danieleverducci@gmail.com>
Date: Sun, 3 Jan 2021 09:37:22 +0100
Subject: [PATCH] Interrupt brings up Monitor: working, but calling "run"
 leaves garbage in the SP

---
 .../z80-assembly/os/abi-generated.asm         | 12 ++---
 .../software/z80-assembly/os/main.asm         |  9 +++-
 .../software/z80-assembly/os/monitor.asm      | 52 +++++++------------
 3 files changed, 32 insertions(+), 41 deletions(-)

diff --git a/pat80-computer/software/z80-assembly/os/abi-generated.asm b/pat80-computer/software/z80-assembly/os/abi-generated.asm
index 675f2f2..984c8ee 100644
--- a/pat80-computer/software/z80-assembly/os/abi-generated.asm
+++ b/pat80-computer/software/z80-assembly/os/abi-generated.asm
@@ -1,6 +1,6 @@
-Sys_ABI:	equ $007e
-Sys_Beep:	equ $008e
-Sys_Print:	equ $0082
-Sys_Printc:	equ $0085
-Sys_Readc:	equ $0088
-Sys_Readline:	equ $008b
+Sys_ABI:	equ $0081
+Sys_Beep:	equ $0091
+Sys_Print:	equ $0085
+Sys_Printc:	equ $0088
+Sys_Readc:	equ $008b
+Sys_Readline:	equ $008e
diff --git a/pat80-computer/software/z80-assembly/os/main.asm b/pat80-computer/software/z80-assembly/os/main.asm
index ee2f2e1..afcf954 100644
--- a/pat80-computer/software/z80-assembly/os/main.asm
+++ b/pat80-computer/software/z80-assembly/os/main.asm
@@ -21,9 +21,14 @@ jp Sysinit     ; Startup vector: DO NOT MOVE! Must be the first instruction
 
 ; **** RESET/INTERRUPT VECTOR ****
 
-; Maskable interrupt mode 1: execute memory monitor
+; 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
-call Monitor_main
+	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).
diff --git a/pat80-computer/software/z80-assembly/os/monitor.asm b/pat80-computer/software/z80-assembly/os/monitor.asm
index f854988..17414bf 100644
--- a/pat80-computer/software/z80-assembly/os/monitor.asm
+++ b/pat80-computer/software/z80-assembly/os/monitor.asm
@@ -31,8 +31,6 @@ MON_DUMP_BYTES_LINES: EQU 8
 MON_DUMP_BYTES_PER_LINE: EQU 8
 
 Monitor_main:
-	; Disable maskable interrupts. MI are used to break a program execution and bring up immediately the memory monitor.
-	call monitor_disable_int
     ; Print welcome string
     ld bc, MON_WELCOME
     call Sys_Print
@@ -92,8 +90,14 @@ monitor_quit:
 	; newline
 	ld a, 10
 	call Sys_Printc
-    ; re-enable interrupts
-	call monitor_enable_int
+    ; 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
 
 
@@ -280,14 +284,18 @@ monitor_load:
 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
-	ld sp, hl ; Point stack pointer to code to execute
-	; re-enable interrupts
-	call monitor_enable_int
-	reti
+    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
@@ -540,28 +548,6 @@ monitor_copyTermToAppMem:
         dec d
         jp monitor_copyTermToAppMem_loop   ; continue loop
 
-; Restores registers and re-enables interrupt.
-; 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).
-; In this way, BREAK key brings up memory monitor at any time.
-; To be called before the user exits from monitor
-monitor_enable_int:
-	; exchange registers
-	exx
-	ex af, af'
-	; enable interrupts
-	ei
-	im 1 ; set interrupt mode 1 (on interrupt jumps to 0x38)
-	ret
-
-; Saves registers and disables interrupts.
-; To be called when the monitor starts
-monitor_disable_int:
-	di ; disable interrupt
-	; exchange registers
-	exx
-	ex af, af'
-	ret
-	
+