diff --git a/PolkaVM/Sources/PolkaVM/Instructions.swift b/PolkaVM/Sources/PolkaVM/Instructions.swift
index e5241120..166eff37 100644
--- a/PolkaVM/Sources/PolkaVM/Instructions.swift
+++ b/PolkaVM/Sources/PolkaVM/Instructions.swift
@@ -187,7 +187,7 @@ public enum Instructions {
         public let offset: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             offset = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -207,7 +207,7 @@ public enum Instructions {
         public let value: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             value = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -224,7 +224,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -242,7 +242,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -260,7 +260,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -281,7 +281,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -302,7 +302,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -323,7 +323,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -341,7 +341,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -359,7 +359,7 @@ public enum Instructions {
         public let address: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             address = try Instructions.decodeImmediate(data.at(relative: 1...))
         }
 
@@ -380,7 +380,7 @@ public enum Instructions {
         public let value: UInt8
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             let (x, y) = try Instructions.decodeImmediate2(data, divideBy: 16)
             address = x
             value = UInt8(truncatingIfNeeded: y)
@@ -400,7 +400,7 @@ public enum Instructions {
         public let value: UInt16
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             let (x, y) = try Instructions.decodeImmediate2(data, divideBy: 16)
             address = x
             value = UInt16(truncatingIfNeeded: y)
@@ -420,7 +420,7 @@ public enum Instructions {
         public let value: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             let (x, y) = try Instructions.decodeImmediate2(data, divideBy: 16)
             address = x
             value = y
@@ -442,7 +442,7 @@ public enum Instructions {
         public let offset: UInt32
 
         public init(data: Data) throws {
-            register = try Registers.Index(data.at(relative: 0))
+            register = try Registers.Index(ra: data.at(relative: 0))
             (value, offset) = try Instructions.decodeImmediate2(data, divideBy: 16)
         }
 
@@ -559,6 +559,45 @@ public enum Instructions {
         var offset: UInt32
         public init(data: Data) throws { (register, value, offset) = try Self.parse(data: data) }
     }
+
+    // MARK: Instructions with Arguments of Two Registers (5.8)
+
+    public struct MoveReg: Instruction {
+        public static var opcode: UInt8 { 82 }
+
+        public let src: Registers.Index
+        public let dest: Registers.Index
+
+        public init(data: Data) throws {
+            dest = try Registers.Index(ra: data.at(relative: 0))
+            src = try Registers.Index(rb: data.at(relative: 0))
+        }
+
+        public func _executeImpl(state: VMState) -> ExecOutcome {
+            state.writeRegister(dest, state.readRegister(src))
+            return .continued
+        }
+    }
+
+    public struct Sbrk: Instruction {
+        public static var opcode: UInt8 { 87 }
+
+        public let src: Registers.Index
+        public let dest: Registers.Index
+
+        public init(data: Data) throws {
+            dest = try Registers.Index(ra: data.at(relative: 0))
+            src = try Registers.Index(rb: data.at(relative: 0))
+        }
+
+        public func _executeImpl(state: VMState) throws -> ExecOutcome {
+            let increment = state.readRegister(src)
+            let startAddr = try state.sbrk(increment)
+            state.writeRegister(dest, startAddr)
+
+            return .continued
+        }
+    }
 }
 
 // MARK: Branch Helpers
@@ -584,7 +623,7 @@ protocol BranchInstructionBase<Compare>: Instruction {
 
 extension BranchInstructionBase {
     public static func parse(data: Data) throws -> (Registers.Index, UInt32, UInt32) {
-        let register = try Registers.Index(data.at(relative: 0))
+        let register = try Registers.Index(ra: data.at(relative: 0))
         let (value, offset) = try Instructions.decodeImmediate2(data, divideBy: 16)
         return (register, value, offset)
     }
diff --git a/PolkaVM/Sources/PolkaVM/Memory.swift b/PolkaVM/Sources/PolkaVM/Memory.swift
index 41a5ee54..1871ef31 100644
--- a/PolkaVM/Sources/PolkaVM/Memory.swift
+++ b/PolkaVM/Sources/PolkaVM/Memory.swift
@@ -4,6 +4,7 @@ public class Memory {
     public enum Error: Swift.Error {
         case pageFault(UInt32)
         case notWritable(UInt32)
+        case outOfMemory(UInt32)
 
         public var address: UInt32 {
             switch self {
@@ -11,6 +12,8 @@ public class Memory {
                 address
             case let .notWritable(address):
                 address
+            case let .outOfMemory(address):
+                address
             }
         }
     }
@@ -129,6 +132,27 @@ public class Memory {
         }
         throw Error.notWritable(address)
     }
+
+    public func sbrk(_ increment: UInt32) throws -> UInt32 {
+        // TODO: update after memory layout details are implemented
+        var curHeapEnd: UInt32 = 0
+
+        if let lastChunk = chunks.last {
+            curHeapEnd = lastChunk.address + UInt32(lastChunk.data.count)
+        }
+
+        for page in pageMap {
+            let pageEnd = page.address + page.length
+            if page.writable, curHeapEnd >= page.address, curHeapEnd + increment < pageEnd {
+                let newChunk = (address: curHeapEnd, data: Data(repeating: 0, count: Int(increment)))
+                chunks.append(newChunk)
+
+                return curHeapEnd
+            }
+        }
+
+        throw Error.outOfMemory(curHeapEnd)
+    }
 }
 
 extension Memory {
diff --git a/PolkaVM/Sources/PolkaVM/Registers.swift b/PolkaVM/Sources/PolkaVM/Registers.swift
index 8ea35a29..dd9a047f 100644
--- a/PolkaVM/Sources/PolkaVM/Registers.swift
+++ b/PolkaVM/Sources/PolkaVM/Registers.swift
@@ -1,8 +1,12 @@
 public struct Registers: Equatable {
     public struct Index {
         public let value: UInt8
-        public init(_ value: UInt8) {
-            self.value = min(value & 0b1111, 12)
+        public init(ra: UInt8) {
+            value = min(ra & 0b1111, 12)
+        }
+
+        public init(rb: UInt8) {
+            value = min(rb >> 4, 12)
         }
     }
 
diff --git a/PolkaVM/Sources/PolkaVM/VMState.swift b/PolkaVM/Sources/PolkaVM/VMState.swift
index 646c73e8..01d68aa1 100644
--- a/PolkaVM/Sources/PolkaVM/VMState.swift
+++ b/PolkaVM/Sources/PolkaVM/VMState.swift
@@ -45,6 +45,10 @@ public class VMState {
         try memory.write(address: address, values: values)
     }
 
+    public func sbrk(_ increment: UInt32) throws -> UInt32 {
+        try memory.sbrk(increment)
+    }
+
     public func consumeGas(_ amount: UInt64) {
         // TODO: use saturating subtraction
         gas -= Int64(amount)