From 9431ca57f4e4258c26abb905cb7408577e57b44a Mon Sep 17 00:00:00 2001
From: Alan Cox <alan@linux.intel.com>
Date: Mon, 25 Sep 2017 15:30:36 +0100
Subject: [PATCH] 65c816: fix various bugs in the core code

With the platform bugs also fixed this takes us to the point that we start
running init and begin to make progress until we take an interrupt in user
mode, at which point badness happens
---
 Kernel/lowlevel-65c816.s    | 54 +++++++++++++++++++++++++------------
 Kernel/usermem_std-65c816.s |  2 +-
 2 files changed, 38 insertions(+), 18 deletions(-)

diff --git a/Kernel/lowlevel-65c816.s b/Kernel/lowlevel-65c816.s
index 4be0465c..13665969 100644
--- a/Kernel/lowlevel-65c816.s
+++ b/Kernel/lowlevel-65c816.s
@@ -299,52 +299,72 @@ signal_out:
 ;
 _doexec:
 	sta	tmp1
-	stx	tmp1+1
+	stx	tmp1+1		;	address to execute from
 	sei
 	stz	_kernel_flag
+
 	rep	#$30
 	.i16
 	.a16
+
 	ldx	tmp1		;	target address
+
 	sep	#$20
 	.a8
+
 	lda	U_DATA__U_PAGE	;	get our bank
-	clc
-	adc	U_DATA__U_PAGE	;	two * bank
+	asl	a
 	adc	#STACK_BANKOFF	;	plus offset
 	xba			;	swap to xx00
 	lda	#$FF		;	stack top is xxff
 	tcs			;	Set CPU stack at xxFF
+
 	rep	#$20
+	.a16
+
 	inc			;	gives us the DP at xx+100
 	pha
 	pld
-	; Split I/D will need to change this logic
-	lda	U_DATA__U_PAGE	;	our bank
-	jsr	setdp
-	; We are now on the correct DP and CPU stack
-	; So fix up the syscall vector
+
+	;
+	;	From this point onwards exec is referencing the user DP
+	;	user S and kernel B (data)
+	;
+
+	sep 	#$20
+	.a8
+
+	; Fix up the syscall vector
+
 	ldy	#syscall_vector
-	sty	syscall
+	sty	syscall		;	stores into user DP
 	; And the C stack pointer
 	ldy	U_DATA__U_ISP
 	sty	sp		;	sp is in DP so we write user version
-	sep	#$30
-	.i8
-	.a8
-	; Will need to change for split I/D
+
+	;
+	;	From here we are entirely referencing user data but kernel
+	;	code
+	;
+	;	(Will need to change for split I/D)
+
 	lda	U_DATA__U_PAGE	;	bank
 	pha			;	switch to userdata bank
 	plb
+
 	pha			;	stack bank for rtl
-	rep	#$10
-	.i16
+
+	dex			;	Points to end of last instruction
+				;	for a return
 	phx			;	Execution address within bank
-	cli
 	lda	#0		;	
 	tay			;	DP base is 0
-	sep	#$10
+
+	sep	#$30
+	.a8
 	.i8
+
+	cli
 	rtl
 
 ;
diff --git a/Kernel/usermem_std-65c816.s b/Kernel/usermem_std-65c816.s
index 1d7c36ed..d42934ef 100644
--- a/Kernel/usermem_std-65c816.s
+++ b/Kernel/usermem_std-65c816.s
@@ -154,7 +154,7 @@ __uput:
 	sta	ptr3
 	stx	ptr3+1
 	lda	U_DATA__U_PAGE
-	sta	uputpatch+2
+	sta	uputpatch+1
 	phb
 	.i16
 	.a16
-- 
2.34.1