Adding README updates and sifter exercise

README.md

@@ -39,7 +39,7 @@ print(s)
 
 Complete this work in [dynamo/program.S](dynamo/program.S).
 
-Our automated station can sense really big space rocks. Like, _sufficiently big_. In this case that means ones measuring `20`-bits. On each rock signal we encounter that is _`20` bits or greater_, we should send out a dynamo to have a look and possibly take some samples so that we can characterize the type and quality of the rocks. (Don't worry: that work is up _next_.)
+Our automated station can sense really big space rocks. Like, _sufficiently big_. In this case that means ones measuring `20 -bits. On each rock signal we encounter that is _`20` bits or greater_, we should send out a dynamo to have a look and possibly take some samples so that we can characterize the type and quality of the rocks. (Don't worry: that work is up _next_.)
 
 However, it's inefficient to do all of this in a long string of instructions. We've got to use what we've learned about the `link register` (`LR`) and stack to make this process as smooth as possible. 
 
@@ -54,6 +54,78 @@ An example outcome should look like:
 ...
 ```
 
+### Lab: The Sifter
+
+Once we get the rocks, we have to classify them. In our current operation, we collect `LUNAR` and `MARTIAN` rocks; then, we sort them into `HI` and `LOW` quality. Here're the rules:
+
+|Number start (base 16)| Type |
+|:---------------------|:-----|
+|`00`                  |`LUNAR`|
+|`55`                  |`MARTIAN`|
+
+|Number end (base 16)| Quality |
+|:---------------------|:-----|
+|`FF`                  |`HIGH`|
+|`00`                  |`LOW`|
+
+> Hints: 
+> * what are these numbers' decimal equivalents?
+> * how can we get only the first `2` bytes and last `2` bytes alone?
+
+### Assignment "Hacks"
+
+See the `Suggestion` below to challenge yourself to implement a Hack. As always, you are allowed to develop
+your own Hack to satisfy this stretch goal. Place the code for the Hack inline with the code in the corresponding
+file.
+
+In order to recieve credit for the Hack, you must fill out the [hack.md](docs/hack.md) file located in the
+`docs` folder.
+
+#### `dynamo`
+
+Negative values denote _very dangerous_ space junk. We have a blaster to demolish it, but it needs some work. However, we're really only concerned
+about negative values above _20 bits_. Anything else will just bounce off our station's force field.
+
+**Note: you will need to use a new _signed number opcode_: `LDRSW`, which loads a _signed word_.**
+
+```
+
+```
+
+Implement the blaster by replacing the `signals` array in `program.S` with the above and print that we've `blasted` the junk
+using a similar format to our dynamo's dispatch message.
+
+> Hint: you may need to engage with the `NEG` or other instruction; shifting may have unpredictable results!
+
+#### `sifter`
+
+Sometimes, as the saying (and reality) goes, sometimes junk gets lodged in a perfectly good space rock. Here, these scraps are denoted by negative numbers. However, sometimes, they contain technology that we might want to salvage -- especially if the most significant bits are `33`. If we encounter
+a negative-signed sample, and it meets our criteria, we need to print the format string:
+
+```assembly
+    salvaged:     .asciz      "0x%08x\tSALVAGED."
+```
+
+Add the above to the `.data` section of the appropriate file.
+
+**Note: you will need to use a new _signed number opcode_: `LDRSW`, which loads a _signed word_.**
+
+##### New files
+
+For any `Hack`, you may choose to create a new file to do this; if you do, make sure you add the new file name to the [CMakeLists.txt](CMakeLists.txt) file in the `add_executable` directive:
+
+```cmake
+add_executable(${PROJECT_NAME}
+    program.S
+    sifter.S
+	your_file_here.S
+)
+```
+
+#### Make it your own
+
+You are free to develop your own "Hack" for this assignment. However, you'll need to be sure to justify the value of your hack clearly!
+
 ### Changes to files in `.vscode`
 
 Based on your system setup (refer to your `hello-blinky` assignment), you will need switch out the `.vscode` folder in each exercise with the _last working copy_.

sifter/.vscode/launch.json

@@ -0,0 +1,31 @@
+{
+    "version": "0.2.0",
+    "configurations": [
+        {   "name": "Pico Debug",
+            "device": "RP2040",
+            "gdbPath": "${env:GDB_PATH}",
+            "cwd": "${workspaceRoot}",
+            "executable": "${command:cmake.launchTargetPath}",
+            "request": "launch",
+            "type": "cortex-debug",
+            "servertype": "openocd",
+            "configFiles": [
+                "interface/cmsis-dap.cfg",
+                "target/rp2040.cfg"
+            ],
+            "searchDir": [
+                "${env:OPENOCD_PATH}/tcl"
+            ],
+            "openOCDLaunchCommands": [
+                "adapter speed 5000"
+            ],
+            "runToEntryPoint": "main",
+            "svdFile": "${env:PICO_SDK_PATH}/src/rp2040/hardware_regs/rp2040.svd",
+            "postRestartCommands": [
+                "break main",
+                "continue"
+            ],
+            "showDevDebugOutput": "parsed"
+        }
+    ]
+}

sifter/.vscode/settings.json

@@ -0,0 +1,23 @@
+{
+    // These settings tweaks to the cmake plugin will ensure
+    // that you debug using cortex-debug instead of trying to launch
+    // a Pico binary on the host
+    "cmake.statusbar.advanced": {
+        "debug": {
+            "visibility": "hidden"
+        },
+        "launch": {
+            "visibility": "hidden"
+        },
+        "build": {
+            "visibility": "hidden"
+        },
+        "buildTarget": {
+            "visibility": "hidden"
+        }
+    },
+    "cmake.buildBeforeRun": true,
+    "C_Cpp.default.configurationProvider": "ms-vscode.cmake-tools",
+    "cortex-debug.openocdPath": "${env:OPENOCD_PATH}/src/openocd",
+    "cortex-debug.variableUseNaturalFormat": false
+}

sifter/CMakeLists.txt

@@ -0,0 +1,27 @@
+cmake_minimum_required(VERSION 3.18)
+include($ENV{PICO_SDK_PATH}\\external\\pico_sdk_import.cmake)
+
+project(program C CXX ASM)
+
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_CXX_STANDARD 17)
+
+include_directories(${CMAKE_SOURCE_DIR})
+
+add_executable(${PROJECT_NAME}
+    program.S
+    sifter.S
+)
+
+set(PICO_BOARD pico_w)
+
+pico_sdk_init()
+
+target_link_libraries(${PROJECT_NAME}
+    pico_stdlib
+)
+
+pico_add_extra_outputs(${PROJECT_NAME})
+
+pico_enable_stdio_usb(${PROJECT_NAME} 1)
+pico_enable_stdio_uart(${PROJECT_NAME} 1)

sifter/program.S

@@ -0,0 +1,25 @@
+.thumb_func		    	@ Necessary because sdk uses BLX
+.global main	   	 	@ Provide program starting address
+
+main:
+    BL          stdio_init_all
+    MOV         R4, #0
+    LDR         R5, =numbers
+    LDR         R6, =size
+
+load:
+    @ TODO: Set up stack frame for load function
+    LDR         R7, [R5, R4]
+    ADD         R4, #4
+    @ TODO: Branch and link to the sifter
+    CMP         R4, R6
+    BNE         load
+
+rest:
+    NOP
+    B           rest
+
+.data
+    numbers:    .word   255, 1426063615, 855638271, 0, 1140850943
+    end:
+    .set        size,   end - numbers

sifter/sifter.S

@@ -0,0 +1,49 @@
+.thumb_func
+.global sifter
+
+sifter:
+    @ TODO: Prepare stack frame for sifter
+    LDR     R0, =format
+    MOV     R1, R7
+    @ TODO: Branch and Link to determine_type
+    @ TODO: Branch and Link to determine_quality
+    BL      printf
+    @ TODO: Pop appropriate registers to...
+    @ TODO: Branch and eXecute back to subroutine calling this one (caller)
+
+determine_type:
+    @ TODO: Prepare stack frame for determine_type
+    @ TODO: Shift appropriate direction(s) to get the type bits
+    @ TODO: Compare and branch to appropriate typing branches
+
+is_lunar_type:
+    LDR     R2, =lunar
+    @ TODO: Pop from stack to get address of caller
+    @ TODO: Branch and eXecute to caller
+
+is_martian_type:
+    LDR     R2, =martian
+    @ TODO: Pop from stack to get address of caller
+    @ TODO: Branch and eXecute to caller
+
+determine_quality:
+    @ TODO: Set up stack frame for determine_quality
+    @ TODO: Shift appropriate places and direction(s) to get quality bits
+    @ TODO: Compare and branch to correct quality branch
+
+hi_quality:
+    LDR     R3, =hi_qual
+    @ TODO: Pop from stack to get address of caller
+    @ TODO: Branch and eXecute to caller
+
+lo_quality:
+    LDR     R3, =lo_qual
+    @ TODO: Pop from stack to get address of caller
+    @ TODO: Branch and eXecute to caller
+
+.data
+    format:     .asciz      "0x%08x\tTYPE: %s\tQUALITY: %s\n"
+    lunar:      .asciz      "LUNAR"
+    martian:    .asciz      "MARTIAN"
+    hi_qual:    .asciz      "HI"
+    lo_qual:    .asciz      "LO"