VBE：為什么我的代碼不提供線性幀緩沖區？

Question

我是一個嘗試在 VBE 中實現簡單圖形的初學者。 我已經編寫了以下匯編代碼來啟動，進入 32 位保護模式，並進入 VBE 模式 0x4117。 （有人告訴我，[mode] OR 0x4000 的 output 會產生一個帶有線性幀緩沖區的模式版本，所以我假設 0x0117 OR 0x4000 = 0x4117 應該有一個線性幀緩沖區。

[org 0x7c00]            ; Origin is same as addr of MBR.
[bits 16]           

section code
switch:
    mov ax, 0x4f01      ; Querying VBE.
    mov cx, 0x4117      ; We want 0x117 mode graphics.
                ; i.e. 16 bits per pixel, 1024x768 res.
    mov bx, 0x0800      ; Offset for VBE info structure.
    mov es, bx
    mov di, 0x00
    int 0x10        ; Graphics interrupt.

    ; Make the switch to graphics mode.
    mov ax, 0x4f02      ; What VBA service is wanted?
                ; 0x4f02 for actual switching.
    mov bx, 0x4117      
    int 0x10

    ; Zero out registers.
    xor ax, ax
    mov ds, ax
    mov es, ax

    ; Here, we call interrupt 13H to read from hard disk.
    mov bx, 0x1000      ; Location where code is loaded from disk.
    mov ah, 0x02        ; Selects the 13H service, in this case
                ; reading sectors from drive.       
    mov al, 30      ; Num sectors to read from hard disk.
                ; We'll make this larger the bigger our OS gets.
    mov ch, 0x00        ; Where is cylinder?
    mov dh, 0x00        ; Where is head?
    mov cl, 0x02        ; Sector.
    int 0x13        ; Call interrupt corresponding to disk services.

    cli         ; Turn off interrupts.
    lgdt [gdt_descriptor]   ; Load global descriptor table.
    
    mov eax, cr0        
    or eax, 0x1
    mov cr0, eax        ; Make switch.

    jmp code_seg:protected_start

text: db "Jesus said I will rebuild this temple in three days. I could make a compiler in 3 days. - Terry A. Davis",0

[bits 32]
protected_start:
    mov ax, data_seg    ; Loads the data segment start ptr from GDT,
    mov ds, ax      ; and set data segment start in program equal.
    mov ss, ax      ; Set stack segment.
    mov es, ax      ; Set extra segment.
    mov fs, ax      ; Set fs (seg. w/ no specific use).
    mov gs, ax      ; Set gs (seg. w/ no specific use).

    mov ebp, 0x90000    ; Update stack ptr to where it's expected.
    mov esp, ebp
    
    call 0x1000     ; Call kernel code which was loaded into 0x1000.
    jmp $

gdt_begin:
gdt_null_descriptor:        ; Null descriptor. Unclear why this is needed.
    dd 0x00
    dd 0x00
gdt_code_seg:
    dw 0xffff       ; Limit of code segment
    dw 0x00         ; Base of code segment.
    db 0x00         ; Base of code segment (con.).
    db 10011010b        ; Acess byte of form:
                ;    - Present (1) - 1 for valid segment.
                ;    - Privl  (2) - 0 for kernel.
                ;    - S (1) - 1 for code/data segment. 
                ;    - Ex (1) - 1 for code segment.
                ;    - Direction bit (1) - 0 for upward growth.
                ;    - RW (1) - 1 for read/writable.
                ;    - Ac (1) - 0 to indicate not accessed yet.

    db 11001111b        ; Split byte.
                ;    - Upper 4 bits are limit (con.), another 0xf.
                ;    - Lower 4 bits are flags in order of:
                ;        - Gr - 1 for 4KiB page granularity.
                ;        - Sz - 1 for 32-bit protected mode.
                ;    - L - 0, since we aren't in long mode.
                ;        - Reserved bit.

    db 0x00         ; Base of code segment (con.).
gdt_data_seg:
    dw 0xffff       ; Limit of data segment.
    dw 0x00         ; Base of data segment.
    db 0x00         ; Base of data segment (con.).
    db 10010010b        ; Acess byte. 
                ; Same as for code segment but Ex=0 for data seg.
    db 11001111b        ; Split byte, same as for code segment.
    db 0x00         ; Base of code segment (con.).
gdt_end:
gdt_descriptor:
    dw gdt_end - gdt_begin - 1  ; GDT limit.
    dd gdt_begin            ; GDT base.

code_seg equ gdt_code_seg - gdt_begin
data_seg equ gdt_data_seg - gdt_begin

times 510 - ($ - $$) db 0x00    ; Pads file w/ 0s until it reaches 512 bytes.

db 0x55
db 0xaa

上面調用了“kernel_entry.asm”，如下圖：

[bits 32]
START:
    [extern start]
    call start      ; Call kernel func from C file.
    jmp $           ; Infinite loop.

反過來，“kernel_entry.asm”調用我的 main.c 文件：

#define PACK_RGB565(r, g, b) \
        (((((r) >> 3) & 0x1f) << 11) | \
         ((((g) >> 2) & 0x3f) << 5) | \
         (((b) >> 3) & 0x1f))

typedef struct VbeInfoBlockStruct {
    unsigned short mode_attribute_;
    unsigned char win_a_attribute_;
    unsigned char win_b_attribute_;
    unsigned short win_granuality_;
    unsigned short win_size_;
    unsigned short win_a_segment_;
    unsigned short win_b_segment_;
    unsigned int win_func_ptr_;
    unsigned short bytes_per_scan_line_;
    unsigned short x_resolution_;
    unsigned short y_resolution_;
    unsigned char char_x_size_;
    unsigned char char_y_size_;
    unsigned char number_of_planes_;
    unsigned char bits_per_pixel_;
    unsigned char number_of_banks_;
    unsigned char memory_model_;
    unsigned char bank_size_;
    unsigned char number_of_image_pages_;
    unsigned char b_reserved_;
    unsigned char red_mask_size_;
    unsigned char red_field_position_;
    unsigned char green_mask_size_;
    unsigned char green_field_position_;
    unsigned char blue_mask_size_;
    unsigned char blue_field_position_;
    unsigned char reserved_mask_size_;
    unsigned char reserved_field_position_;
    unsigned char direct_color_info_;
    unsigned int screen_ptr_;
} VbeInfoBlock;

// VBE Info block will be located at this address at boot time.
#define VBE_INFO_ADDR 0x8000

int start()
{
    VbeInfoBlock *gVbe = (VbeInfoBlock*) VBE_INFO_ADDR;
    for(int i = 0; i < gVbe->y_resolution_; ++i) {
        for(int j = 0; j < gVbe->x_resolution_; ++j) {
            unsigned long offset = i * gVbe->y_resolution_ + j;
            *((unsigned short*) gVbe->screen_ptr_ + offset) = PACK_RGB565(0,i,j);
        }
    }
}

如果我正確加載了線性幀緩沖區，我希望看到漸變。 相反，我看到了這個：

一系列盒子，每個盒子里都有一個漸變，它突然切斷了。 這似乎表明我正在使用存儲幀緩沖區而不是線性緩沖區的模式進行編寫； 梯度走出一個緩沖區，持續數百次迭代，最終到達下一個緩沖區的開始，導致突然移位和“盒子”效應。

我的解釋正確嗎？ 我是否正確加載了線性幀緩沖區，如果沒有，我該怎么做？

編輯：我試過改變unsigned long offset = i * gVbe->y_resolution_ + j; 到unsigned long offset = i * gVbe->bytes_per_scan_line_ + j ，如下面的小丑建議。 這產生了以下圖像。 它同樣是四四方方的。

Answer 1

我是否正確加載了線性幀緩沖區，如果沒有，我該怎么做？

在您的代碼中，您只是假設線性幀緩沖區模式可用。 您應該檢查 ModeInfoBlock.ModeAttributes 位 7 以確定。 該位需要打開：

    mov  ax, 0x4F01      ; Querying VBE.
    mov  cx, 0x0117      ; We want 0x117 mode graphics.
                         ; i.e. 16 bits per pixel, 1024x768 res.
    mov  bx, 0x0800      ; Offset for VBE info structure.
    mov  es, bx
    mov  di, 0x00
    int  0x10
    mov  al, [es:di]
    test al, al
    jns  NoLFB           ; Bit 7 is not set!
                         ; Make the switch to graphics mode.
    mov  ax, 0x4F02      ; What VBA service is wanted?
    mov  bx, 0x4117      
    int  0x10

由於此視頻模式每個像素使用 2 個字節，因此視頻 memory 中的偏移量的計算需要將 x 坐標翻倍：

unsigned long offset = (i * gVbe->bytes_per_scan_line_) + (j * 2)

提示：為什么不使用x和y而不是j和i ； 為了清楚...