diff --git a/README.md b/README.md
index e502673..82dc10e 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ Writing a "bare metal" operating system for Raspberry Pi 4
 Introduction
 ------------
 
-As a tech CEO @RealVNC, I don't write code any more. And I've recently realised just how much I miss it.
+As a tech CEO, I don't write code any more. And I've recently realised just how much I miss it.
 
 Currently in the throes of a nationwide "lockdown" due to Covid-19 (and having been spared my usual commute), I've found myself with more hours in the day. I have taken this time for myself and used it to fulfil a childhood ambition - to write a **bare metal** operating system that runs on commercial hardware.
 
diff --git a/part10-multicore/io.c b/part10-multicore/io.c
index a2a888d..b9ba71f 100644
--- a/part10-multicore/io.c
+++ b/part10-multicore/io.c
@@ -19,8 +19,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part11-breakout-smp/Makefile.gcc b/part11-breakout-smp/Makefile.gcc
index 9c0526f..023aa68 100644
--- a/part11-breakout-smp/Makefile.gcc
+++ b/part11-breakout-smp/Makefile.gcc
@@ -1,9 +1,8 @@
 CFILES = $(wildcard *.c lib/*.c)
 OFILES = $(CFILES:.c=.o)
-GCCFLAGS = -Wall -O2 -ffreestanding -nostdinc -nostdlib
+GCCFLAGS = -Wall -O2 -ffreestanding -nostdinc -nostdlib -nostartfiles
 GCCPATH = ../../gcc-arm-10.3-2021.07-x86_64-aarch64-none-elf/bin
 
-
 all: clean kernel8.img
 
 boot/boot.o: boot/boot.S
diff --git a/part11-breakout-smp/lib/io.c b/part11-breakout-smp/lib/io.c
index 34a02aa..77e23eb 100644
--- a/part11-breakout-smp/lib/io.c
+++ b/part11-breakout-smp/lib/io.c
@@ -19,8 +19,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part12-wgt/lib/io.c b/part12-wgt/lib/io.c
index 34a02aa..77e23eb 100644
--- a/part12-wgt/lib/io.c
+++ b/part12-wgt/lib/io.c
@@ -19,8 +19,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part13-interrupts/lib/io.c b/part13-interrupts/lib/io.c
index 34a02aa..77e23eb 100644
--- a/part13-interrupts/lib/io.c
+++ b/part13-interrupts/lib/io.c
@@ -19,8 +19,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part14-spi-ethernet/lib/io.c b/part14-spi-ethernet/lib/io.c
index 34a02aa..77e23eb 100644
--- a/part14-spi-ethernet/lib/io.c
+++ b/part14-spi-ethernet/lib/io.c
@@ -19,8 +19,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part4-miniuart/io.c b/part4-miniuart/io.c
index 9561780..d7b2985 100644
--- a/part4-miniuart/io.c
+++ b/part4-miniuart/io.c
@@ -17,8 +17,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part5-framebuffer/io.c b/part5-framebuffer/io.c
index e8af328..b58f996 100644
--- a/part5-framebuffer/io.c
+++ b/part5-framebuffer/io.c
@@ -18,8 +18,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part6-breakout/io.c b/part6-breakout/io.c
index e06b64d..c5134ab 100644
--- a/part6-breakout/io.c
+++ b/part6-breakout/io.c
@@ -18,8 +18,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part7-bluetooth/io.c b/part7-bluetooth/io.c
index 8a591b7..33b3962 100644
--- a/part7-bluetooth/io.c
+++ b/part7-bluetooth/io.c
@@ -18,8 +18,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part8-breakout-ble/io.c b/part8-breakout-ble/io.c
index 8a591b7..33b3962 100644
--- a/part8-breakout-ble/io.c
+++ b/part8-breakout-ble/io.c
@@ -18,8 +18,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }
diff --git a/part9-sound/io.c b/part9-sound/io.c
index a2a888d..b9ba71f 100644
--- a/part9-sound/io.c
+++ b/part9-sound/io.c
@@ -19,8 +19,8 @@ enum {
 
 enum {
     Pull_None = 0,
-    Pull_Down = 1, // Are down and up the right way around?
-    Pull_Up = 2
+    Pull_Down = 2,
+    Pull_Up = 1
 };
 
 void mmio_write(long reg, unsigned int val) { *(volatile unsigned int *)reg = val; }