// Registers according to Ultimate 64 Command Interface description,
// https://1541u-documentation.readthedocs.io/en/latest/command%20interface.html
// and ultimate_dos_v1.0.pdf and command_interface_v1.0.pdf by Gideon Zweijtzer

	// Command interface registers
	.label U64_CONTROL_STATUS          = $DF1C // write = CONTROL, read = STATUS
	.label U64_COMMAND_DATA_IDENTIFY   = $DF1D // write = COMMAND_DATA, read = IDENTIFICATION
	.label U64_RESPONSE_DATA           = $DF1E // read only
	.label U64_STATUS_DATA             = $DF1F // read only

	// Constant for U64_IDENTIFICATION register
	.const U64_MAGIC_ID                = $C9

	// Constants for U64_CONTROL register
	.const U64_CTRL_BIT_PUSH_CMD       = $01
	.const U64_CTRL_BIT_DATA_ACC       = $02
	.const U64_CTRL_BIT_ABORT          = $04
	.const U64_CTRL_BIT_CLR_ERR        = $08

	// Constants for U64_STATUS register
	.const U64_STAT_BIT_CMD_BUSY       = $01
	.const U64_STAT_BIT_DATA_ACC       = $02
	.const U64_STAT_BIT_ABORT_P        = $04
	.const U64_STAT_BIT_ERROR          = $08
	.const U64_STAT_MASK_STATE         = $30 // two bits!
	.const U64_STAT_BIT_STAT_AV        = $40
	.const U64_STAT_BIT_DATA_AV        = $80

	// Interface targets
	.const U64_TARGET_DOS1             = $01
	.const U64_TARGET_DOS2             = $02

	// DOS command codes
	.const U64_DOS_CMD_IDENTIFY        = $01
	.const U64_DOS_CMD_OPEN_FILE       = $02
	.const U64_DOS_CMD_CLOSE_FILE      = $03
	.const U64_DOS_CMD_READ_DATA       = $04
	.const U64_DOS_CMD_WRITE_DATA      = $05
	.const U64_DOS_CMD_FILE_SEEK       = $06
	.const U64_DOS_CMD_FILE_INFO       = $07
	.const U64_DOS_CMD_CHANGE_DIR      = $11
	.const U64_DOS_CMD_GET_PATH        = $12
	.const U64_DOS_CMD_OPEN_DIR        = $13
	.const U64_DOS_CMD_READ_DIR        = $14
	.const U64_DOS_CMD_COPY_UI_PATH    = $15
	.const U64_DOS_CMD_LOAD_REU        = $21
	.const U64_DOS_CMD_SAVE_REU        = $22
	.const U64_DOS_CMD_ECHO            = $F0

	// DOS file opening constants 
	.const U64_FA_READ                 = $01
	.const U64_FA_WRITE                = $02
	.const U64_FA_CREATE_NEW           = $03
	.const U64_FA_CREATE_ALWAYS        = $04