OVMF Print 输出 - 技术栈

c 复制代码

  for (SrcY = SourceY, DstY = DestinationY;
       SrcY < (Height + SourceY);
       SrcY++, DstY++)
  {
    Offset      = (DstY * Configure->PixelsPerScanLine) + DestinationX;
    Offset      = Configure->BytesPerPixel * Offset;
    Destination = Configure->FrameBuffer + Offset;

    if (Configure->PixelFormat == PixelBlueGreenRedReserved8BitPerColor) {
      Source = (UINT8 *)BltBuffer + (SrcY * Delta) + SourceX * sizeof (EFI_GRAPHICS_OUTPUT_BLT_PIXEL);
    } else {
      for (IndexX = 0; IndexX < Width; IndexX++) {
        Blt =
          (EFI_GRAPHICS_OUTPUT_BLT_PIXEL *)(
                                            (UINT8 *)BltBuffer +
                                            (SrcY * Delta) +
                                            ((SourceX + IndexX) * sizeof (EFI_GRAPHICS_OUTPUT_BLT_PIXEL))
                                            );
        Uint32                                                                   = *(UINT32 *)Blt;
        *(UINT32 *)(Configure->LineBuffer + (IndexX * Configure->BytesPerPixel)) =
          (UINT32)(
                   (((Uint32 << Configure->PixelShl[0]) >> Configure->PixelShr[0]) &
                    Configure->PixelMasks.RedMask) |
                   (((Uint32 << Configure->PixelShl[1]) >> Configure->PixelShr[1]) &
                    Configure->PixelMasks.GreenMask) |
                   (((Uint32 << Configure->PixelShl[2]) >> Configure->PixelShr[2]) &
                    Configure->PixelMasks.BlueMask)
                   );
      }

      Source = Configure->LineBuffer;
    }

    CopyMem (Destination, Source, WidthInBytes);
  }

nasm 复制代码

global InternalMemCopyMem
InternalMemCopyMem:
    push    rsi
    push    rdi
    mov     rsi, rdx                    ; rsi <- Source
    mov     rdi, rcx                    ; rdi <- Destination
    lea     r9, [rsi + r8 - 1]          ; r9 <- End of Source
    cmp     rsi, rdi
    mov     rax, rdi                    ; rax <- Destination as return value
    jae     .0
    cmp     r9, rdi
    jae     @CopyBackward               ; Copy backward if overlapped
.0:
    mov     rcx, r8
    and     r8, 7
    shr     rcx, 3
    rep     movsq                       ; Copy as many Qwords as possible
    jmp     @CopyBytes
@CopyBackward:
    mov     rsi, r9                     ; rsi <- End of Source
    lea     rdi, [rdi + r8 - 1]         ; esi <- End of Destination
    std                                 ; set direction flag
@CopyBytes:
    mov     rcx, r8
    rep     movsb                       ; Copy bytes backward
    cld
    pop     rdi
    pop     rsi
    ret

print 在屏幕上的输出通过把数据写入显存实现

rep movsq 是一条硬件级内存块拷贝指令
rep ：重复前缀，表示重复执行后续指令直到 RCX 寄存器为 0
movsq ：移动四字，每次拷贝 8 字节

寄存器隐式依赖

寄存器	角色	说明
`RCX`	计数器	要重复的次数
`RSI`	源地址	数据从哪里拷贝
`RDI`	目标地址	数据拷贝到哪里
`DF` 标志	方向	0=递增（默认），1=递减