diff options
| author | Marvin Borner | 2021-07-05 12:59:37 +0200 |
|---|---|---|
| committer | Marvin Borner | 2021-07-05 12:59:37 +0200 |
| commit | 832a5046b42f53f7ab329942a862e1b4c0a6f6db (patch) | |
| tree | 33133b077696710b6db8a2689f3dce06d1b98c73 | |
| parent | 0c4427cada337b4a80e1ec0a16be08b75412321b (diff) | |
Reduced GDT magic numbers
| -rw-r--r-- | src/entry/bootsector.asm | 9 | ||||
| -rw-r--r-- | src/entry/definitions.asm | 12 |
2 files changed, 17 insertions, 4 deletions
diff --git a/src/entry/bootsector.asm b/src/entry/bootsector.asm index 1beb850..c3532ee 100644 --- a/src/entry/bootsector.asm +++ b/src/entry/bootsector.asm @@ -139,6 +139,7 @@ dap: ; Disk Address Packet dd 0 ; More storage bytes ; Global Descriptor Table (GDT) +; There are better explanations for the constants in definitions.asm gdt: dw .size - 1 + 8 ; GDT size dd .start - 8 ; GDT start address @@ -147,16 +148,16 @@ gdt: dw 0xffff ; Limit dw 0x0000 ; Base (low 16 bits) db 0x00 ; Base (mid 8 bits) - db 10011010b ; Access - db 11001111b ; Granularity + db GDT_PRESENT | GDT_DESCRIPTOR | GDT_EXECUTABLE | GDT_READWRITE ; Access + db GDT_GRANULARITY | GDT_SIZE | 0xf ; Granularity db 0x00 ; Base (high 8 bits) ; Data dw 0xffff ; Limit dw 0x0000 ; Base (low 16 bits) db 0x00 ; Base (mid 8 bits) - db 10010010b ; Access - db 11001111b ; Granularity + db GDT_PRESENT | GDT_DESCRIPTOR | GDT_READWRITE ; Access; Data isn't executable + db GDT_GRANULARITY | GDT_SIZE | 0xf ; Granularity db 0x00 ; Base (high 8 bits) .end: .size: equ .end - .start diff --git a/src/entry/definitions.asm b/src/entry/definitions.asm index dcaa141..019eefa 100644 --- a/src/entry/definitions.asm +++ b/src/entry/definitions.asm @@ -24,3 +24,15 @@ %define DISK_EXT_CHECK_REQ 0x55aa ; First extension check signature (request) %define DISK_EXT_CHECK_RESP 0xaa55 ; Second extension check signature (response) %define DISK_READ 0x42 ; Disk extended read command + +; GDT constants (bitmap) +%define GDT_MAX_LIMIT 0xffff ; I just use the max limit lel +%define GDT_PRESENT 0b10000000 ; Is present +%define GDT_RING3 0b01100000 ; Privilege level 3 +%define GDT_DESCRIPTOR 0b00010000 ; Descriptor type, set for code/data +%define GDT_EXECUTABLE 0b00001000 ; Can be executed +%define GDT_READWRITE 0b00000010 ; Read/write access for code/data +%define GDT_ACCESSED 0b00000001 ; Whether segment is accessible +%define GDT_GRANULARITY 0x80 ; Page granularity (4KiB) +%define GDT_SIZE 0x40 ; Use 32 bit selectors +%define GDT_DATA_OFFSET 0x10 ; Offset to GDT data segment |