From f3992ed59855f3cfcfa0e785765b7055997d380e Mon Sep 17 00:00:00 2001
From: Virgil Dupras <hsoft@hardcoded.net>
Date: Wed, 13 Nov 2019 15:28:16 -0500
Subject: [PATCH] basic: begin an implementation from sratch

Let's see where it will lead us...
---
 apps/README.md        |  7 +++++
 apps/basic/README.md  | 16 ++++++++++
 apps/basic/glue.asm   | 17 ++++++++++
 apps/basic/main.asm   | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++
 tools/emul/.gitignore |  1 +
 tools/emul/Makefile   |  2 +-
 6 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 apps/basic/README.md
 create mode 100644 apps/basic/glue.asm
 create mode 100644 apps/basic/main.asm

diff --git a/apps/README.md b/apps/README.md
index 7be6e2e..f40fe27 100644
--- a/apps/README.md
+++ b/apps/README.md
@@ -18,3 +18,10 @@ HL to a pointer to unparsed arguments in string form, null terminated.
 
 The userspace application is expected to set A on return. 0 means success,
 non-zero means error.
+
+A userspace application can expect the `SP` pointer to be properly set. If it
+moves it, it should take care of returning it where it was before returning
+because otherwise, it will break the kernel.
+
+Apps in Collapse OS are design to be ROM-compatible, that is, they don't write
+to addresses that are part of the code's address space.
diff --git a/apps/basic/README.md b/apps/basic/README.md
new file mode 100644
index 0000000..d2c38c2
--- /dev/null
+++ b/apps/basic/README.md
@@ -0,0 +1,16 @@
+# basic
+
+**Work in progress, not finished.**
+
+This is a BASIC interpreter which is being written from scratch for Collapse OS.
+There are many existing z80 implementations around, some of them open source
+and most of them good and efficient, but because a lot of that code overlaps
+with code that has already been written for zasm, I believe that it's better to
+reuse those bits of code.
+
+Integrating an existing BASIC to Collapse OS seemed a bigger challenge than
+writing from scratch, so here I am, writing from scratch again...
+
+The biggest challenge here is to extract code from zasm, adapt it to fit BASIC,
+not break anything, and have the wisdom to see when copy/pasting is a better
+idea.
diff --git a/apps/basic/glue.asm b/apps/basic/glue.asm
new file mode 100644
index 0000000..e7216bc
--- /dev/null
+++ b/apps/basic/glue.asm
@@ -0,0 +1,17 @@
+; *** Requirements ***
+; addHL
+; printstr
+; printcrlf
+; stdioReadLine
+; strncmp
+;
+.inc "user.h"
+
+.inc "err.h"
+.org	USER_CODE
+
+	jp	basStart
+
+.inc "lib/parse.asm"
+.equ	BAS_RAMSTART	USER_RAMSTART
+.inc "basic/main.asm"
diff --git a/apps/basic/main.asm b/apps/basic/main.asm
new file mode 100644
index 0000000..e281716
--- /dev/null
+++ b/apps/basic/main.asm
@@ -0,0 +1,87 @@
+; *** Variables ***
+; Value of `SP` when basic was first invoked. This is where SP is going back to
+; on restarts.
+.equ	BAS_INITSP	BAS_RAMSTART
+; **Pointer** to current line number
+.equ	BAS_PCURLN	@+2
+.equ	BAS_RAMEND	@+2
+
+; *** Code ***
+basStart:
+	ld	(BAS_INITSP), sp
+	xor	a
+	ld	hl, .welcome
+	call	printstr
+	call	printcrlf
+	ld	hl, .welcome+2		; points to a zero word
+	ld	(BAS_PCURLN), hl
+	jr	basPrompt
+
+.welcome:
+	.db "OK", 0, 0
+
+basPrompt:
+	ld	hl, .sPrompt
+	call	printstr
+	call	stdioReadLine
+	call	parseDecimal
+	jr	z, .number
+	call	basDirect
+	jr	basPrompt
+.number:
+	; do nothing for now, we only support direct mode.
+	ld	hl, .sNumber
+	call	basPrintLn
+	jr	basPrompt
+.sNumber:
+	.db "A number!", 0
+.sPrompt:
+	.db "> ", 0
+
+basDirect:
+	ex	de, hl
+	ld	hl, basCmds1
+.loop:
+	ld	a, 4
+	call	strncmp
+	jr	z, .found
+	ld	a, 6
+	call	addHL
+	ld	a, (hl)
+	cp	0xff
+	jr	nz, .loop
+	ld	hl, .sUnknown
+	jr	basPrintLn
+
+.found:
+	inc	hl \ inc hl \ inc hl \ inc hl
+	call	intoHL
+	jp	(hl)
+
+.sUnknown:
+	.db	"Unknown command", 0
+
+basPrintLn:
+	call	printcrlf
+	call	printstr
+	jp	printcrlf
+
+; *** Commands ***
+basBYE:
+	ld	hl, .sBye
+	call	basPrintLn
+	; To quit the loop, let's return the stack to its initial value and
+	; then return.
+	xor	a
+	ld	sp, (BAS_INITSP)
+	ret
+.sBye:
+	.db	"Goodbye!", 0
+
+; direct only
+basCmds1:
+	.db	"bye", 0
+	.dw	basBYE
+; statements
+basCmds2:
+	.db	0xff	; end of table
diff --git a/tools/emul/.gitignore b/tools/emul/.gitignore
index cc529d7..d4bb0a3 100644
--- a/tools/emul/.gitignore
+++ b/tools/emul/.gitignore
@@ -4,4 +4,5 @@
 /*/*-bin.h
 /cfsin/zasm
 /cfsin/ed
+/cfsin/basic
 /cfsin/user.h
diff --git a/tools/emul/Makefile b/tools/emul/Makefile
index 69511f3..affef32 100644
--- a/tools/emul/Makefile
+++ b/tools/emul/Makefile
@@ -4,7 +4,7 @@ KERNEL = ../../kernel
 APPS = ../../apps
 ZASMBIN = zasm/zasm
 ZASMSH = ../zasm.sh
-SHELLAPPS = $(addprefix cfsin/, zasm ed)
+SHELLAPPS = $(addprefix cfsin/, zasm ed basic)
 CFSIN_CONTENTS = $(SHELLAPPS) cfsin/user.h
 
 .PHONY: all