Editing TI-89 series (section)

=== Hardware versions ===
There are four hardware versions of the TI-89. These versions are normally referred to as HW1, HW2, HW3, and HW4 (released in May 2006).  Entering the key sequence '''[F1] [A]''' displays the hardware version.  Older versions (before HW2) don't display anything about the hardware version in the about menu.  The differences in the hardware versions are not well documented by Texas Instruments.  HW1 and HW2 correspond to the original TI-89; HW3 and HW4 are only present in the TI-89 Titanium.

The most significant difference between HW1 and HW2 is in the way the calculator handles the display. In HW1 calculators there is a video buffer that stores all of the information that should be displayed on the screen, and every time the screen is refreshed the calculator accesses this buffer and flushes it to the display ([[direct memory access]]).  In HW2 and later calculators, a region of memory is directly aliased to the display controller ([[memory-mapped I/O]]).  This allows for slightly faster memory access, as the HW1's DMA controller used about 10% of the bus bandwidth.  However, it interferes with a trick some programs use to implement [[grayscale]] graphics by rapidly switching between two or more displays (page-flipping).  On the HW1, the DMA controller's base address can be changed (a single write into a [[Memory-mapped I/O|memory-mapped]] [[hardware register]]) and the screen will automatically use a new section of memory at the beginning of the next frame.  In HW2, the new page must be written to the screen by software. The effect of this is to cause increased flickering in grayscale mode, enough to make the 7-level grayscale supported on the HW1 unusable (although 4-level grayscale works on both calculators).

HW2 calculators are slightly faster because TI increased the nominal speed of the processor from 10&nbsp;MHz to 12&nbsp;MHz.  It is believed that TI increased the speed of HW4 calculators to 16&nbsp;MHz, though many users disagree about this finding. The measured statistics are closer to 14&nbsp;MHz.

Another difference between HW1 and HW2 calculators is assembly program size limitations. The size limitation on HW2 calculators has varied with the AMS version of the calculator. As of AMS 2.09 the limit is 24k.  Some earlier versions limited assembly programs to 8k, and the earliest AMS versions had no limit. The latest AMS version has a 64kb limit. HW1 calculators have no hardware to enforce the limits, so it is easy to bypass them in software.  There are [[unofficial patch]]es and kernels that can be installed on HW2 calculators to remove the limitations.