previous Macs, along with backward compatibility. The drive is controlled by the same IWM chip as in previous models, implementing variable speed GCR. The drive was still completely incompatible with PC drives. The 800 KB drive has two read/write heads, enabling it to simultaneously use both sides of the floppy disk and thereby double storage capacity. Like the 400 KB drive before it, a companion Macintosh 800K External Drive was an available option. However, with the increased disk storage capacity combined with 2-4x the available RAM, the external drive was less of a necessity than it had been with the 128K and 512K. The Mac Plus has 128 KB of ROM on the motherboard, which is double the amount of ROM in previous Macs; the ROMs included software to support SCSI, the then-new 800 KB floppy drive, and the Hierarchical File System (HFS), which uses a true directory structure on disks (as opposed to the earlier MFS, Macintosh File System in which all files were stored in a single directory, with one level of pseudo-folders overlaid on them). For programmers, the fourth Inside Macintosh volume details how to use HFS and the rest of the Mac Plus's new system software. The Plus still did not include provision for an internal hard drive and it would be over nine months before Apple would offer a SCSI drive replacement for the slow Hard Disk 20. It would be well over a year before Apple would offer the first internal hard disk drive in any Macintosh. A compact Mac, the Plus has a 512 × 342 pixel monochrome display with a resolution of 72 PPI, identical to that of previous Macintosh models. Unlike earlier Macs, the Mac Plus's keyboard includes a numeric keypad and directional arrow keys and, as with previous Macs, it has a one-button mouse and no fan, making it extremely quiet in operation. The lack of a cooling fan in the Mac Plus led to frequent problems with overheating and hardware malfunctions. The applications MacPaint and MacWrite were bundled with the Mac Plus. After August 1987, HyperCard and MultiFinder were also bundled. Third-party software applications available included MacDraw, Microsoft Word, Excel, and PowerPoint, as well as Aldus PageMaker. Microsoft Excel and PowerPoint (originally by Forethought) were actually developed and released first for the Macintosh, and similarly Microsoft Word 1 for Macintosh was the first time a GUI version of that