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81 lines
2.2 KiB
ArmAsm
81 lines
2.2 KiB
ArmAsm
; Reads sectors from disk
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; Args:
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; eax - LBA low 32 bits
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; ebp - LBA high 32 bits
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; dl - Drive number
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; es - Destination segment
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; bx - Destination offset
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; ecx - Byte count
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; Rets:
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; CF - Set on error
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read_sectors:
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pusha
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; Fill out Disk Address Packet
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mov si, .da_packet
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mov word [si], 0x10 ; Packet Size and Unused
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mov word [si+2], 1 ; Sector count
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mov word [si+4], bx ; Dest. offset
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mov word [si+6], es ; Dest. segment
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; Store some regs for later
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push dx
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push si
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push eax
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push ebp
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; Get bytes per sector
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mov ah, 0x48 ; Extended read drive parameters
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mov si, .drive_params ; DS:SI - result buffer
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mov word [si], 0x1e ; Buffer size
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int 0x13
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jc .done ; CF set - error; we never pop ebp, eax, si & dx! (could cause issues)
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mov bp, word [si+24] ; Bytes per sector
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; Convert byte count in ECX to sector count in CX
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mov ax, cx ; Low word
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shr ecx, 16 ; Shift high word into low word of ECX (therefore just CX)
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mov dx, cx ; High word
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xor cx, cx ; Clear CX
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div bp ; Divide byte count in ax (low word) by bytes per sector
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test dx, dx ; DX will contain remainder
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setnz cl ; (dx != 0) ? cl = 1 : cl = 0
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add cx, ax ; Add remaining sectors to cx
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pop edx ; Restore pushed EBP to EDX
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pop eax ; Restore EAX
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pop si ; Restore SI
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; Calculate destination LBA
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div ebp ; Divide EAX by EBP
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mov dword [si+8], eax ; LBA low
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mov dword [si+12], 0 ; LBA high
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pop dx ; Restore DX
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; Read sectors one by one
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.loop:
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mov ah, 0x42 ; Extended read
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clc ; Clear CF
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int 0x13
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jc .done ; Error
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add word [si+4], bp ; Increment destination by sector byte count
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xor ebx, ebx ; EBX = 0
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inc dword [si+8] ; Increment LBA low
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seto bl ; Will set BL to 1 in case of overflow
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add dword [si+12], ebx ; Increment LBA high
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loop .loop ; dec cx; jnz .loop
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.done:
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popa
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ret
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; Disk Address Packet
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.da_packet: equ 0x8000
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; Drive Parameters from int 13h, ah = 0x48
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.drive_params: equ 0x8010
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