Fixed "jc fail" instructions not working properly and updated README.md (#453)

* Fixed "jc fail" instructions not working properly and updated README.md

* Fixed stage 2 fail calls and removed additional asm label

Author: spencer3035

bios/boot_sector/README.md

@@ -4,13 +4,13 @@ This executable needs to fit into the 512-byte boot sector, so we need to use al
 
 ## Build Commands
 
-1. `cargo build --release -Zbuild-std=core --target x86-16bit.json -Zbuild-std-features=compiler-builtins-mem`
-2. `objcopy -I elf32-i386 -O binary target/x86-16bit/release/first_stage target/disk_image.bin
+1. `cargo build --profile=stage-1 -Zbuild-std=core --target ../../i386-code16-boot-sector.json -Zbuild-std-features=compiler-builtins-mem`
+2. `objcopy -I elf32-i386 -O binary ../../target/i386-code16-boot-sector/stage-1/bootloader-x86_64-bios-boot-sector ../../target/disk_image.img`
 
 To run in QEMU:
 
-- `qemu-system-x86_64 -drive format=raw,file=target/disk_image.bin`
+- `qemu-system-x86_64 -drive format=raw,file=../../target/disk_image.img`
 
 To print the contents of the ELF file, e.g. for trying to bring the size down:
 
-- `objdump -xsdS -M i8086,intel target/x86-16bit/release/first_stage`
+- `objdump -xsdS -M i8086,intel ../../target/i386-code16-boot-sector/stage-1/bootloader-x86_64-bios-boot-sector`

bios/boot_sector/src/boot.s

@@ -36,13 +36,18 @@ check_int13h_extensions:
     mov bx, 0x55aa
     # dl contains drive number
     int 0x13
-    jc fail
+    jnc .int13_pass
+    call fail
+.int13_pass:
     pop ax      # pop error code again
 
 rust:
     # push arguments
     push dx     # disk number
     call first_stage
+    # Fail code if first stage returns
+    push 'x'
+    call fail
 
 spin:
     hlt

bios/boot_sector/src/dap.rs

@@ -38,11 +38,13 @@ impl DiskAddressPacket {
         let self_addr = self as *const Self as u16;
         unsafe {
             asm!(
-                "push 0x7a", // error code `z`, passed to `fail` on error
+                "push 'z'", // error code `z`, passed to `fail` on error
                 "mov {1:x}, si", // backup the `si` register, whose contents are required by LLVM
                 "mov si, {0:x}",
                 "int 0x13",
-                "jc fail",
+                "jnc 2f", // carry is set on fail
+                "call fail",
+                "2:",
                 "pop si", // remove error code again
                 "mov si, {1:x}", // restore the `si` register to its prior state
                 in(reg) self_addr,

bios/boot_sector/src/mbr.rs

@@ -16,14 +16,14 @@ pub(crate) fn get_partition(partitions_raw: &[u8], index: usize) -> PartitionTab
             .get(8..)
             .and_then(|s| s.get(..4))
             .and_then(|s| s.try_into().ok())
-            .unwrap_or_fail(b'e'),
+            .unwrap_or_fail(b'f'),
     );
     let len = u32::from_le_bytes(
         buffer
             .get(12..)
             .and_then(|s| s.get(..4))
             .and_then(|s| s.try_into().ok())
-            .unwrap_or_fail(b'f'),
+            .unwrap_or_fail(b'g'),
     );
     PartitionTableEntry::new(bootable, partition_type, lba, len)
 }

bios/stage-2/src/dap.rs

@@ -38,11 +38,13 @@ impl DiskAddressPacket {
     pub unsafe fn perform_load(&self, disk_number: u16) {
         let self_addr = self as *const Self as u16;
         asm!(
-            "push 0x7a", // error code `z`, passed to `fail` on error
+            "push 'z'", // error code `z`, passed to `fail` on error
             "mov {1:x}, si",
             "mov si, {0:x}",
             "int 0x13",
-            "jc fail",
+            "jnc 2f", // carry is set on fail
+            "call fail",
+            "2:",
             "pop si", // remove error code again
             "mov si, {1:x}",
             in(reg) self_addr,