diff --git a/prog/dump/isaset.8 b/prog/dump/isaset.8
index bb1c3b6a..a659b928 100644
--- a/prog/dump/isaset.8
+++ b/prog/dump/isaset.8
@@ -9,12 +9,14 @@ i2cset \- set ISA registers
 .I datareg
 .I address
 .I value
+.RI [ mask ]
 #for I2C-like access
 .br
 .B isaset
 .RB [ -y ]
 .BI "-f " address
 .I value
+.RI [ mask ]
 #for flat address space
 
 .SH DESCRIPTION
@@ -33,24 +35,27 @@ will perform the operation directly. This is mainly meant to be used in
 scripts.
 
 .SH OPTIONS (I2C-like access mode)
-Exactly four options must be provided to isaset. \fIaddrreg\fR contains the
+Four options must be provided to isaset. \fIaddrreg\fR contains the
 ISA address of the address register for the chip to probe; \fIdatareg\fR
 contains the address of the data register. Both addresses are integers between
 0x0000 and 0x3FFF. Usually, if the chip's base address is 0x0nn0, the
 address register is at 0x0nn5 and the data register is at 0x0nn6. The most
 common base address for hardware monitoring chips is 0x0290.
-.PP
 For Super-I/O chips, address register is typically at 0x2E with data
 register at 0x2F.
 The \fIaddress\fR and \fIvalue\fR parameters are two integers between
 0x00 and 0xFF. isaset will write value \fIvalue\fR to address \fIaddress\fR.
+An optional \fImask\fR can be provided as a fifth parameter, preserving
+unmasked bits at the written location.
 
 .SH OPTIONS (flat address space mode)
-In flat mode, exactly two parameters must
+In flat mode, two parameters must
 be provided. \fIaddress\fR contains the ISA address of the register to
 write to; it is an integer between 0x0000 and 0xFFFF. Basically, it is
 the sum of the chip's base address and the chip register's address. isaset
 will write value \fIvalue\fR at this address.
+An optional \fImask\fR can be provided as a third parameter, preserving
+unmasked bits at the written location.
 
 .SH WARNING
 Poking around in ISA data space is extremely dangerous.
diff --git a/prog/dump/isaset.c b/prog/dump/isaset.c
index c1dad73e..6f591381 100644
--- a/prog/dump/isaset.c
+++ b/prog/dump/isaset.c
@@ -46,14 +46,14 @@ void help(void)
 {
 	fprintf(stderr,
 	        "Syntax for I2C-like access:\n"
-	        "  isaset [-y] ADDRREG DATAREG ADDRESS VALUE\n"
+	        "  isaset [-y] ADDRREG DATAREG ADDRESS VALUE [MASK]\n"
 	        "Syntax for flat address space:\n"
-	        "  isaset [-y] -f ADDRESS VALUE\n");
+	        "  isaset [-y] -f ADDRESS VALUE [MASK]\n");
 }
 
 int main(int argc, char *argv[])
 {
-	int addrreg, datareg = 0, value, addr = 0;
+	int addrreg, datareg = 0, value, addr = 0, vmask = 0;
 	unsigned char res;
 	int flags = 0;
 	int flat = 0, yes = 0;
@@ -74,8 +74,8 @@ int main(int argc, char *argv[])
 	}
 
 	/* verify that the argument count is correct */
-	if ((!flat && argc != 1+flags+4)
-	 || (flat && argc != 1+flags+2)) {
+	if ((!flat && (argc < 1+flags+4 || argc > 1+flags+5))
+	 || (flat && (argc < 1+flags+2 || argc > 1+flags+3))) {
 		help();
 		exit(1);
 	}
@@ -122,7 +122,11 @@ int main(int argc, char *argv[])
 		}
 	}
 
-	value = strtol(argv[flat?1+flags+1:1+flags+3], &end, 0);
+	/* rest is the same for both modes so we cheat on flags */
+	if (!flat)
+		flags += 2;
+
+	value = strtol(argv[flags+2], &end, 0);
 	if (*end) {
 		fprintf(stderr, "Error: Invalid value!\n");
 		help();
@@ -135,6 +139,21 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
+	if (flags+3 < argc) {
+		vmask = strtol(argv[flags+3], &end, 0);
+		if (*end) {
+			fprintf(stderr, "Error: Invalid mask!\n");
+			help();
+			exit(1);
+		}
+		if (vmask < 0 || vmask > 0xff) {
+			fprintf(stderr, "Error: Mask out of range "
+				"(0x00-0xff)!\n");
+			help();
+			exit(1);
+		}
+	}
+
 	if (getuid()) {
 		fprintf(stderr, "Error: Can only be run as root "
 		        "(or make it suid root)\n");
@@ -148,13 +167,15 @@ int main(int argc, char *argv[])
 		        "system crashes, data loss and worse!\n");
 
 		if (flat)
-			fprintf(stderr, "I will write value 0x%02x to address "
-		                "0x%x.\n", value, addrreg);
+			fprintf(stderr, "I will write value 0x%02x%s to address "
+		                "0x%x.\n", value, vmask ? " (masked)" : "",
+			        addrreg);
 		else
-			fprintf(stderr, "I will write value 0x%02x to address "
+			fprintf(stderr, "I will write value 0x%02x%s to address "
 		                "0x%02x of chip with address register 0x%x\n"
 		                "and data register 0x%x.\n",
-		                value, addr, addrreg, datareg);
+		                value, vmask ? " (masked)" : "", addr,
+			        addrreg, datareg);
 
 		fprintf(stderr, "Continue? [Y/n] ");
 		fflush(stderr);
@@ -185,19 +206,53 @@ int main(int argc, char *argv[])
 	}
 #endif
 
-	/* write */
-	if (flat) {
-		outb(value, addrreg);
-	} else {	
-		outb(addr, addrreg);
-		outb(value, datareg);
+	if (vmask) {
+		int oldvalue;
+
+		if (flat) {
+			oldvalue = inb(addrreg);
+		} else {	
+			outb(addr, addrreg);
+			oldvalue = inb(datareg);
+		}
+
+		if (oldvalue < 0) {
+			fprintf(stderr, "Error: Failed to read old value\n");
+			exit(1);
+		}
+
+		value = (value & vmask) | (oldvalue & ~vmask);
+
+		if (!yes) {
+			char s[2];
+			
+			fprintf(stderr, "Old value 0x%02x, write mask "
+				"0x%02x: Will write 0x%02x to %s "
+				"0x%02x\n", oldvalue, vmask, value,
+				flat ? "address" : "register",
+				flat ? addrreg : addr);
+
+			fprintf(stderr, "Continue? [Y/n] ");
+			fflush(stderr);
+			fgets(s, 2, stdin);
+			if (s[0] != '\n' && s[0] != 'y' && s[0] != 'Y') {
+				fprintf(stderr, "Aborting on user request.\n");
+				exit(0);
+			}
+		}
 	}
 
-	/* readback */
+	/* do the real thing */
 	if (flat) {
+		/* write */
+		outb(value, addrreg);
+		/* readback */
 		res = inb(addrreg);
 	} else {	
+		/* write */
 		outb(addr, addrreg);
+		outb(value, datareg);
+		/* readback */
 		res = inb(datareg);
 	}