Add zasm app

For now, only a dummy app, but it's emulated properly with libz80.
Exciting times!

apps/README.md

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+# User applications
+
+This folder contains code designed to be loaded and ran in RAM.

apps/zasm/README.md

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+# z80 assembler
+
+This is probably the most critical part of the Collapse OS project. If this app
+can be brought to completion, it pretty much makes the project a success because
+it ensures self-reproduction.
+
+## Running on a "modern" machine
+
+To be able to develop zasm efficiently, [libz80][libz80] is used to run zasm
+on a modern machine. The code lives in `emul` and ran be built with `make`,
+provided that you have a copy libz80 living in `emul/libz80`.
+
+The resulting `zasm` binary takes asm code in stdin and spits binary in stdout.
+
+[libz80]: https://github.com/ggambetta/libz80

apps/zasm/emul/.gitignore

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+libz80
+wrapper.h
+zasm.h
+zasm

apps/zasm/emul/Makefile

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+zasm: zasm.c libz80/libz80.so wrapper.h zasm.h
+	cc $< -l z80 -L./libz80 -Wl,-rpath ./libz80 -o $@
+
+libz80/libz80.so: libz80/Makefile
+	make -C libz80
+
+wrapper.h: wrapper.asm
+	scas -o - $< | ./bin2c.sh WRAPPER > $@
+
+zasm.h: ../zasm.asm
+	scas -o - $< | ./bin2c.sh ZASM > $@

apps/zasm/emul/bin2c.sh

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+#!/bin/sh
+
+echo "unsigned char $1[] = { "
+xxd -i -
+echo " };"

apps/zasm/emul/wrapper.asm

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+; setup the stack
+ld	hl, 0xffff
+ld	sp, hl
+; zasm input
+ld	hl, 0x9000
+; zasm output
+ld	hl, 0xc000
+call	zasm
+; signal the emulator we're done
+out	(0), a
+halt
+zasm:
+; beginning of the code

apps/zasm/emul/zasm.c

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+#include <stdint.h>
+#include "libz80/z80.h"
+#include "wrapper.h"
+#include "zasm.h"
+
+/* zasm is a "pure memory" application. It starts up being told memory location
+ * to read and memory location to write.
+ *
+ * This program works be writing stdin in a specific location in memory, run
+ * zasm in a special wrapper, wait until we receive the stop signal, then
+ * spit the contents of the dest memory to stdout.
+ */
+static Z80Context cpu;
+static uint8_t mem[0xffff];
+static int running;
+
+
+static uint8_t io_read(int unused, uint16_t addr)
+{
+    return 0;
+}
+
+static void io_write(int unused, uint16_t addr, uint8_t val)
+{
+    // zasm doesn't do any IO. If we receive any IO, it means that we're done
+    // because the wrapper told us through an "out"
+    running = 0;
+}
+
+static uint8_t mem_read(int unused, uint16_t addr)
+{
+    return mem[addr];
+}
+
+static void mem_write(int unused, uint16_t addr, uint8_t val)
+{
+    mem[addr] = val;
+}
+
+int main()
+{
+    // initialize memory
+    int wrapperlen = sizeof(WRAPPER);
+    for (int i=0; i<wrapperlen; i++) {
+        mem[i] = WRAPPER[i];
+    }
+    int zasm = sizeof(ZASM);
+    for (int i=0; i<zasm; i++) {
+        mem[i+wrapperlen] = ZASM[i];
+    }
+    // Run!
+    running = 1;
+    Z80RESET(&cpu);
+    cpu.ioRead = io_read;
+    cpu.ioWrite = io_write;
+    cpu.memRead = mem_read;
+    cpu.memWrite = mem_write;
+
+    while (running) {
+        Z80Execute(&cpu);
+    }
+    printf("and... %d!\n", mem[0x100]);
+    return 0;
+}

apps/zasm/zasm.asm

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+; dummy code, to test emulator
+ld	a, 0x42
+ld	(0x100), a
+ret