diff --git a/evm/src/generation/state.rs b/evm/src/generation/state.rs
index 4dbd90fe..1b26a3c4 100644
--- a/evm/src/generation/state.rs
+++ b/evm/src/generation/state.rs
@@ -62,7 +62,7 @@ impl<F: Field> GenerationState<F> {
         let context = self.current_context;
         let value = self.memory.contexts[context].segments[segment].get(virt);
         self.memory.log.push(MemoryOp {
-            channel_index,
+            channel_index: Some(channel_index),
             timestamp,
             is_read: true,
             context,
@@ -84,7 +84,7 @@ impl<F: Field> GenerationState<F> {
         let timestamp = self.cpu_rows.len();
         let context = self.current_context;
         self.memory.log.push(MemoryOp {
-            channel_index,
+            channel_index: Some(channel_index),
             timestamp,
             is_read: false,
             context,
diff --git a/evm/src/memory/columns.rs b/evm/src/memory/columns.rs
index 65e75891..5f6c3911 100644
--- a/evm/src/memory/columns.rs
+++ b/evm/src/memory/columns.rs
@@ -1,7 +1,5 @@
 //! Memory registers.
 
-use std::ops::Range;
-
 use crate::memory::{NUM_CHANNELS, VALUE_LIMBS};
 
 // Columns for memory operations, ordered by (addr, timestamp).
@@ -41,7 +39,4 @@ pub(crate) const COUNTER: usize = RANGE_CHECK + 1;
 pub(crate) const RANGE_CHECK_PERMUTED: usize = COUNTER + 1;
 pub(crate) const COUNTER_PERMUTED: usize = RANGE_CHECK_PERMUTED + 1;
 
-// Columns to be padded at the top with zeroes, before the permutation argument takes place.
-pub(crate) const COLUMNS_TO_PAD: Range<usize> = TIMESTAMP..RANGE_CHECK + 1;
-
 pub(crate) const NUM_COLUMNS: usize = COUNTER_PERMUTED + 1;
diff --git a/evm/src/memory/memory_stark.rs b/evm/src/memory/memory_stark.rs
index 1653ac16..14a75810 100644
--- a/evm/src/memory/memory_stark.rs
+++ b/evm/src/memory/memory_stark.rs
@@ -15,11 +15,11 @@ use crate::constraint_consumer::{ConstraintConsumer, RecursiveConstraintConsumer
 use crate::cross_table_lookup::Column;
 use crate::lookup::{eval_lookups, eval_lookups_circuit, permuted_cols};
 use crate::memory::columns::{
-    is_channel, value_limb, ADDR_CONTEXT, ADDR_SEGMENT, ADDR_VIRTUAL, COLUMNS_TO_PAD,
-    CONTEXT_FIRST_CHANGE, COUNTER, COUNTER_PERMUTED, IS_READ, NUM_COLUMNS, RANGE_CHECK,
-    RANGE_CHECK_PERMUTED, SEGMENT_FIRST_CHANGE, TIMESTAMP, VIRTUAL_FIRST_CHANGE,
+    is_channel, value_limb, ADDR_CONTEXT, ADDR_SEGMENT, ADDR_VIRTUAL, CONTEXT_FIRST_CHANGE,
+    COUNTER, COUNTER_PERMUTED, IS_READ, NUM_COLUMNS, RANGE_CHECK, RANGE_CHECK_PERMUTED,
+    SEGMENT_FIRST_CHANGE, TIMESTAMP, VIRTUAL_FIRST_CHANGE,
 };
-use crate::memory::NUM_CHANNELS;
+use crate::memory::{NUM_CHANNELS, VALUE_LIMBS};
 use crate::permutation::PermutationPair;
 use crate::stark::Stark;
 use crate::util::trace_rows_to_poly_values;
@@ -44,9 +44,10 @@ pub struct MemoryStark<F, const D: usize> {
     pub(crate) f: PhantomData<F>,
 }
 
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct MemoryOp<F> {
-    pub channel_index: usize,
+    /// The channel this operation came from, or `None` if it's a dummy operation for padding.
+    pub channel_index: Option<usize>,
     pub timestamp: usize,
     pub is_read: bool,
     pub context: usize,
@@ -111,7 +112,7 @@ pub fn generate_random_memory_ops<F: RichField, R: Rng>(
 
             let timestamp = clock * NUM_CHANNELS + channel_index;
             memory_ops.push(MemoryOp {
-                channel_index,
+                channel_index: Some(channel_index),
                 timestamp,
                 is_read,
                 context,
@@ -128,6 +129,25 @@ pub fn generate_random_memory_ops<F: RichField, R: Rng>(
     memory_ops
 }
 
+fn get_max_range_check<F: Field>(memory_ops: &[MemoryOp<F>]) -> usize {
+    memory_ops
+        .iter()
+        .tuple_windows()
+        .map(|(curr, next)| {
+            if curr.context != next.context {
+                next.context - curr.context - 1
+            } else if curr.segment != next.segment {
+                next.segment - curr.segment - 1
+            } else if curr.virt != next.virt {
+                next.virt - curr.virt - 1
+            } else {
+                next.timestamp - curr.timestamp - 1
+            }
+        })
+        .max()
+        .unwrap_or(0)
+}
+
 pub fn generate_first_change_flags<F: RichField>(
     context: &[F],
     segment: &[F],
@@ -169,7 +189,7 @@ pub fn generate_range_check_value<F: RichField>(
     context_first_change: &[F],
     segment_first_change: &[F],
     virtual_first_change: &[F],
-) -> (Vec<F>, usize) {
+) -> Vec<F> {
     let num_ops = context.len();
     let mut range_check = Vec::new();
 
@@ -187,9 +207,7 @@ pub fn generate_range_check_value<F: RichField>(
     }
     range_check.push(F::ZERO);
 
-    let max_diff = range_check.iter().map(F::to_canonical_u64).max().unwrap() as usize;
-
-    (range_check, max_diff)
+    range_check
 }
 
 impl<F: RichField + Extendable<D>, const D: usize> MemoryStark<F, D> {
@@ -198,7 +216,7 @@ impl<F: RichField + Extendable<D>, const D: usize> MemoryStark<F, D> {
         mut memory_ops: Vec<MemoryOp<F>>,
     ) -> Vec<[F; NUM_COLUMNS]> {
         memory_ops.sort_by_key(|op| (op.context, op.segment, op.virt, op.timestamp));
-
+        Self::pad_memory_ops(&mut memory_ops);
         let num_ops = memory_ops.len();
 
         let mut trace_cols = [(); NUM_COLUMNS].map(|_| vec![F::ZERO; num_ops]);
@@ -212,22 +230,21 @@ impl<F: RichField + Extendable<D>, const D: usize> MemoryStark<F, D> {
                 virt,
                 value,
             } = memory_ops[i];
-            trace_cols[is_channel(channel_index)][i] = F::ONE;
+            if let Some(channel) = channel_index {
+                trace_cols[is_channel(channel)][i] = F::ONE;
+            }
             trace_cols[TIMESTAMP][i] = F::from_canonical_usize(timestamp);
             trace_cols[IS_READ][i] = F::from_bool(is_read);
             trace_cols[ADDR_CONTEXT][i] = F::from_canonical_usize(context);
             trace_cols[ADDR_SEGMENT][i] = F::from_canonical_usize(segment);
             trace_cols[ADDR_VIRTUAL][i] = F::from_canonical_usize(virt);
-            for j in 0..8 {
+            for j in 0..VALUE_LIMBS {
                 trace_cols[value_limb(j)][i] = value[j];
             }
         }
 
         self.generate_memory(&mut trace_cols);
 
-        // The number of rows may have changed, if the range check required padding.
-        let num_ops = trace_cols[0].len();
-
         let mut trace_rows = vec![[F::ZERO; NUM_COLUMNS]; num_ops];
         for (i, col) in trace_cols.iter().enumerate() {
             for (j, &val) in col.iter().enumerate() {
@@ -237,6 +254,29 @@ impl<F: RichField + Extendable<D>, const D: usize> MemoryStark<F, D> {
         trace_rows
     }
 
+    fn pad_memory_ops(memory_ops: &mut Vec<MemoryOp<F>>) {
+        let num_ops = memory_ops.len();
+        let max_range_check = get_max_range_check(&memory_ops);
+        let num_ops_padded = num_ops.max(max_range_check + 1).next_power_of_two();
+        let to_pad = num_ops_padded - num_ops;
+
+        let last_op = memory_ops.last().expect("No memory ops?").clone();
+
+        // We essentially repeat the last operation until our operation list has the desired size,
+        // with a few changes:
+        // - We change its channel to `None` to indicate that this is a dummy operation.
+        // - We increment its timestamp in order to pass the ordering check.
+        // - We make sure it's a read, sine dummy operations must be reads.
+        for i in 0..to_pad {
+            memory_ops.push(MemoryOp {
+                channel_index: None,
+                timestamp: last_op.timestamp + i + 1,
+                is_read: true,
+                ..last_op
+            });
+        }
+    }
+
     fn generate_memory(&self, trace_cols: &mut [Vec<F>]) {
         let num_trace_rows = trace_cols[0].len();
 
@@ -248,7 +288,7 @@ impl<F: RichField + Extendable<D>, const D: usize> MemoryStark<F, D> {
         let (context_first_change, segment_first_change, virtual_first_change) =
             generate_first_change_flags(context, segment, virtuals);
 
-        let (range_check_value, max_diff) = generate_range_check_value(
+        trace_cols[RANGE_CHECK] = generate_range_check_value(
             context,
             segment,
             virtuals,
@@ -257,20 +297,14 @@ impl<F: RichField + Extendable<D>, const D: usize> MemoryStark<F, D> {
             &segment_first_change,
             &virtual_first_change,
         );
-        let to_pad_to = (max_diff + 1).max(num_trace_rows).next_power_of_two();
-        let to_pad = to_pad_to - num_trace_rows;
 
         trace_cols[CONTEXT_FIRST_CHANGE] = context_first_change;
         trace_cols[SEGMENT_FIRST_CHANGE] = segment_first_change;
         trace_cols[VIRTUAL_FIRST_CHANGE] = virtual_first_change;
 
-        trace_cols[RANGE_CHECK] = range_check_value;
-
-        for col in COLUMNS_TO_PAD {
-            trace_cols[col].splice(0..0, vec![F::ZERO; to_pad]);
-        }
-
-        trace_cols[COUNTER] = (0..to_pad_to).map(|i| F::from_canonical_usize(i)).collect();
+        trace_cols[COUNTER] = (0..num_trace_rows)
+            .map(|i| F::from_canonical_usize(i))
+            .collect();
 
         let (permuted_inputs, permuted_table) =
             permuted_cols(&trace_cols[RANGE_CHECK], &trace_cols[COUNTER]);
@@ -326,11 +360,23 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for MemoryStark<F
         let next_addr_virtual = vars.next_values[ADDR_VIRTUAL];
         let next_values: Vec<_> = (0..8).map(|i| vars.next_values[value_limb(i)]).collect();
 
-        // Indicator that this is a real row, not a row of padding.
-        // TODO: enforce that all padding is at the beginning.
-        let valid_row: P = (0..NUM_CHANNELS)
+        // Each `is_channel` value must be 0 or 1.
+        for c in 0..NUM_CHANNELS {
+            let is_channel = vars.local_values[is_channel(c)];
+            yield_constr.constraint(is_channel * (is_channel - P::ONES));
+        }
+
+        // The sum of `is_channel` flags, `has_channel`, must also be 0 or 1.
+        let has_channel: P = (0..NUM_CHANNELS)
             .map(|c| vars.local_values[is_channel(c)])
             .sum();
+        yield_constr.constraint(has_channel * (has_channel - P::ONES));
+
+        // If this is a dummy row (with no channel), it must be a read. This means the prover can
+        // insert reads which never appear in the CPU trace (which are harmless), but not writes.
+        let is_dummy = P::ONES - has_channel;
+        let is_write = P::ONES - vars.local_values[IS_READ];
+        yield_constr.constraint(is_dummy * is_write);
 
         let context_first_change = vars.local_values[CONTEXT_FIRST_CHANGE];
         let segment_first_change = vars.local_values[SEGMENT_FIRST_CHANGE];
@@ -358,21 +404,15 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for MemoryStark<F
             .constraint_transition(virtual_first_change * (next_addr_context - addr_context));
         yield_constr
             .constraint_transition(virtual_first_change * (next_addr_segment - addr_segment));
-        yield_constr.constraint_transition(
-            valid_row * address_unchanged * (next_addr_context - addr_context),
-        );
-        yield_constr.constraint_transition(
-            valid_row * address_unchanged * (next_addr_segment - addr_segment),
-        );
-        yield_constr.constraint_transition(
-            valid_row * address_unchanged * (next_addr_virtual - addr_virtual),
-        );
+        yield_constr.constraint_transition(address_unchanged * (next_addr_context - addr_context));
+        yield_constr.constraint_transition(address_unchanged * (next_addr_segment - addr_segment));
+        yield_constr.constraint_transition(address_unchanged * (next_addr_virtual - addr_virtual));
 
         // Third set of ordering constraints: range-check difference in the column that should be increasing.
         let computed_range_check = context_first_change * (next_addr_context - addr_context - one)
             + segment_first_change * (next_addr_segment - addr_segment - one)
             + virtual_first_change * (next_addr_virtual - addr_virtual - one)
-            + valid_row * address_unchanged * (next_timestamp - timestamp - one);
+            + address_unchanged * (next_timestamp - timestamp - one);
         yield_constr.constraint_transition(range_check - computed_range_check);
 
         // Enumerate purportedly-ordered log.
@@ -405,12 +445,26 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for MemoryStark<F
         let next_is_read = vars.next_values[IS_READ];
         let next_timestamp = vars.next_values[TIMESTAMP];
 
-        // Indicator that this is a real row, not a row of padding.
-        let mut valid_row = vars.local_values[is_channel(0)];
-        for c in 1..NUM_CHANNELS {
-            valid_row = builder.add_extension(valid_row, vars.local_values[is_channel(c)]);
+        // Each `is_channel` value must be 0 or 1.
+        for c in 0..NUM_CHANNELS {
+            let is_channel = vars.local_values[is_channel(c)];
+            let constraint = builder.mul_sub_extension(is_channel, is_channel, is_channel);
+            yield_constr.constraint(builder, constraint);
         }
 
+        // The sum of `is_channel` flags, `has_channel`, must also be 0 or 1.
+        let has_channel =
+            builder.add_many_extension((0..NUM_CHANNELS).map(|c| vars.local_values[is_channel(c)]));
+        let has_channel_bool = builder.mul_sub_extension(has_channel, has_channel, has_channel);
+        yield_constr.constraint(builder, has_channel_bool);
+
+        // If this is a dummy row (with no channel), it must be a read. This means the prover can
+        // insert reads which never appear in the CPU trace (which are harmless), but not writes.
+        let is_dummy = builder.sub_extension(one, has_channel);
+        let is_write = builder.sub_extension(one, vars.local_values[IS_READ]);
+        let is_dummy_write = builder.mul_extension(is_dummy, is_write);
+        yield_constr.constraint(builder, is_dummy_write);
+
         let context_first_change = vars.local_values[CONTEXT_FIRST_CHANGE];
         let segment_first_change = vars.local_values[SEGMENT_FIRST_CHANGE];
         let virtual_first_change = vars.local_values[VIRTUAL_FIRST_CHANGE];
@@ -455,17 +509,11 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for MemoryStark<F
             builder.mul_extension(virtual_first_change, addr_segment_diff);
         yield_constr.constraint_transition(builder, virtual_first_change_check_2);
         let address_unchanged_check_1 = builder.mul_extension(address_unchanged, addr_context_diff);
-        let address_unchanged_check_1_valid =
-            builder.mul_extension(valid_row, address_unchanged_check_1);
-        yield_constr.constraint_transition(builder, address_unchanged_check_1_valid);
+        yield_constr.constraint_transition(builder, address_unchanged_check_1);
         let address_unchanged_check_2 = builder.mul_extension(address_unchanged, addr_segment_diff);
-        let address_unchanged_check_2_valid =
-            builder.mul_extension(valid_row, address_unchanged_check_2);
-        yield_constr.constraint_transition(builder, address_unchanged_check_2_valid);
+        yield_constr.constraint_transition(builder, address_unchanged_check_2);
         let address_unchanged_check_3 = builder.mul_extension(address_unchanged, addr_virtual_diff);
-        let address_unchanged_check_3_valid =
-            builder.mul_extension(valid_row, address_unchanged_check_3);
-        yield_constr.constraint_transition(builder, address_unchanged_check_3_valid);
+        yield_constr.constraint_transition(builder, address_unchanged_check_3);
 
         // Third set of ordering constraints: range-check difference in the column that should be increasing.
         let context_diff = {
@@ -488,7 +536,6 @@ impl<F: RichField + Extendable<D>, const D: usize> Stark<F, D> for MemoryStark<F
             builder.sub_extension(diff, one)
         };
         let timestamp_range_check = builder.mul_extension(address_unchanged, timestamp_diff);
-        let timestamp_range_check = builder.mul_extension(valid_row, timestamp_range_check);
 
         let computed_range_check = {
             let mut sum = builder.add_extension(context_range_check, segment_range_check);