7.17 SYS_READ (0x06)

Reads the contents of a file into a buffer.

The file position is specified either:

  • explicitly by a SYS_SEEK

  • implicitly one byte beyond the previous SYS_READ or SYS_WRITE request.

The file position is at the start of the file when it is opened, and is lost when the file is closed. Perform the file operation as a single action whenever possible. For example, do not split a read of 16KB into four 4KB chunks unless there is no alternative.

Entry

On entry, R1 contains a pointer to a four-word data block:

word 1

contains a handle for a file previously opened with SYS_OPEN

word 2

points to a buffer

word 3

contains the number of bytes to read to the buffer from the file.

Return

On exit:

  • R0 contains zero if the call is successful.

  • If R0 contains the same value as word 3, the call has failed and EOF is assumed.

  • If R0 contains a smaller value than word 3, the call was partially successful. No error is assumed, but the buffer has not been filled.

If the handle is for an interactive device, that is, SYS_ISTTY returns –1. A nonzero return from SYS_READ indicates that the line read did not fill the buffer.

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