diff --git a/common/common.h b/common/common.h
index 16eec7f..0ba1150 100644
--- a/common/common.h
+++ b/common/common.h
@@ -47,7 +47,7 @@
 ; CMR pq		0f pq		F <- Rp <=> Rq	- compare registers
 
 ; 40 bytes in page zero for common registers
-_R0	= $b8
+_R0	= $100 - 4 * (10 + 9)
 _R1	= _R0 + 4
 _R2	= _R1 + 4
 _R3	= _R2 + 4
@@ -58,15 +58,16 @@ _R7	= _R6 + 4
 _R8	= _R7 + 4
 _R9	= _R8 + 4
 
-; 32 bytes in page zero for internal registers
-_I0	= $e0				; workspace
+; 36 bytes in page zero for internal registers
+_I0	= _R9 + 4			; workspace
 _I1	= _I0 + 4
 _I2	= _I1 + 4
 _I3	= _I2 + 4
 _I4	= _I3 + 4
 _I5	= _I4 + 4
 _I6	= _I5 + 4			; register I6 maintains common status
-_I7	= _I6 + 4			; register I7 saves/restores processor status
+_I7	= _I6 + 4			; register I7 maintains process information for context switching
+_I8	= _I7 + 4			; register I8 saves/restores processor status
 
 ; register I6 maintains common status
 ; (dd cc bb aa) aa: index for register stack RS / ccbb: program counter PC / dd: flags F UONPZLGE
@@ -86,21 +87,33 @@ _F_N	=  32				; if Rr < 0.0 (after TST)
 _F_O	=  64				; if overflow (after arithmetic operations)
 _F_U	= 128				; if underflow (after arithmetic operations)
 
-; register I7 saves/restores processor status
+; register I7 maintains process information for context switching
+_PST	= _I7				; current process status
+_PSI	= _PST + 1			; process stack index to save/restore
+_PD0	= _PSI + 1			; two bytes for process
+_PD1	= _PD0 + 1
+
+; register I8 saves/restores processor status
 ; (dd cc bb aa) aa: accumulator, bb: index X, cc: index Y, dd: processor status
-_ACC	= _I7				; saved accumulator to restore
+_ACC	= _I8				; saved accumulator to restore
 _IDX	= _ACC + 1			; saved index X to restore
 _IDY	= _IDX + 1			; saved index Y to restore
 _PS	= _IDY + 1			; saved processor status to restore
 
-; 224 bytes of page two
+; 6502 stack resides on page one
+
+; using some of page two for register stack
 _RS	= $200				; register stack
-_RSS	= (FN_FX - _RS)			; register stack size
+_RSS	= _R0				; register stack size
 
-; for context switching, at least _R0 to FN_FX-1 plus one byte for the stack pointer need to be saved and restored
+; for context switching, _R0 to _RS + _RSS - 1 needs to be saved and restored
+; this should comprise two pages or 512 bytes
 
-; last 32 bytes of page two
-FN_FX	= $2e0				; list of system and user functions
+_PR	= _RS + _RSS			; running processes
+_PRS	= FN_FX - _PR			; running process size
+
+; 32 bytes of page two
+FN_FX	= $300 - 2 * 16			; list of system and user functions
 
 ; function constants
 _ESC_C	= $00
diff --git a/emulator/main.c b/emulator/main.c
index 7aa7202..2a53ac3 100644
--- a/emulator/main.c
+++ b/emulator/main.c
@@ -2,11 +2,15 @@
 #include <stdio.h>
 #include "emulator.h"
 
-/* Register E maintains common status */
-#define _RE	0xf8
+/* Register I6 maintains common status */
+#define _R0	0xb4
+#define _R8	0xd4
+#define _I0	0xdc
+#define _I6	0xf4
+#define _I8	0xfc
 
 /* (dd cc bb aa) aa: index for register stack RS / ccbb: program counter PC / dd: flags F UONPZLGE */
-#define _RSI	_RE		/* register stack index */
+#define _RSI	_I6		/* register stack index */
 #define _PCL	_RSI + 1	/* program counter low */
 #define _PCH	_PCL + 1	/* program counter high */
 #define _F	_PCH + 1	/* flags */
@@ -26,18 +30,18 @@ void hook() {
 	int i, j;
 
 	printf("\n%04x %u %u\n", pc, instructions, clockticks6502);
-	for (i = 0x00b8; i < 0x00d8; i += 4) {
-		printf("R%d: ", (i - 0x00b8) / 4);
+	for (i = _R0; i < _R8; i += 4) {
+		printf("R%d: ", (i - _R0) / 4);
 		for (j = 0; j < 4; ++j)
 			printf("%02x ", memory[i + j]);
-		if (((i - 0x00b8) / 4) % 4 == 3)
+		if (((i - _R0) / 4) % 4 == 3)
 			printf("\n");
 	}
-	for (i = 0x00e0; i < 0x0100; i += 4) {
-		printf("I%d: ", (i - 0x00e0) / 4);
+	for (i = _I0; i < _I8; i += 4) {
+		printf("I%d: ", (i - _I0) / 4);
 		for (j = 0; j < 4; ++j)
 			printf("%02x ", memory[i + j]);
-		if (((i - 0x00e0) / 4) % 4 == 3)
+		if (((i - _I0) / 4) % 4 == 3)
 			printf("\n");
 	}
 }