Chapter 14 – More about PRINT and INPUT

Coordinate Systems

Before we go into more detail about how we can exercise a bit more control on PRINT and INPUT it is useful to understand a little bit about the way NextBASIC views character positioning on the screen. Due to the requirements for backwards compatibility with previous Sinclair computers, NextBASIC uses two distinct coordinate systems to keep track of where text is input or outputted. The first –or legacy– system is based on a virtual matrix that exists on screen and organises it in rigid rows and columns. The second, is more precise and allows for freely positioned columns and rows along the x and y-axes. Additionally the legacy coordinate system has been extended to allow for direct manipulation of the footer bar and status area for layers other than Layer 0, which is not normally possible in the legacy system.

Screen Modes and Pixel Coordinates

In order to make a concrete distinction between the two coordinate systems, we should first discuss a little bit about the ZX Spectrum Next's display system. We will revisit this again in Chapters 15 and 16 in more detail as these chapters deal with the full graphics capabilities of the computer rather than the subset dedicated to screen character manipulation, but for now let's enumerate the screen modes in a simple fashion.

The ZX Spectrum Next has 12 distinct graphics modes broken into 4 groups –or layers– with an additional Sprite Layer which we will not be covering in this chapter . These modes are accessed using the LAYER command with the exception of Layer 3 (Character Graphics) and the higher resolution Layer 2 modes and they are the following:

• Layer 0
  • Layer 0 – Standard Spectrum (ULA) mode, 256 w x 192 h pixels, 8 colours total (2 intensities), 32 x 24 cells, each capable of displaying 2 colours
• Layer 1
  • Layer 1, 0 – LoRes (EnhancedULA) mode, 128 w x 96 h pixels, 256 colours total, 1 colour per pixel
  • Layer 1, 1 – Standard Res (EnhancedULA) mode, 256 w x 192 h pixels, 256 colours total, 32 x 24 cells, each capable of displaying 2 colours
  • Layer 1, 2 – Timex HiRes (EnhancedULA) mode, 512 w x 192 h pixels, 256 colours total, only 2 colours on screen
  • Layer 1, 3 – Timex HiColour (EnhancedULA) mode, 256 w x 192 h pixels, 256 colours total, 32 x 192 cells, each capable of displaying 2 colours
• Layer 2
  • Layer 2 – 256 w x 192 h pixels, 256 colours total, one colour per pixel
  • Layer 2,2 – 320 w x 256 h pixels, 256 colours total, one colour per pixel
  • Layer 2,3 – 640 w x 256 h pixels, 16 colours total, one colour per pixel
• Layer 3
  • Layer 3,0 – Text mode, 320 w x 256 h pixels, 256 colours total, 40 x 32 cells each capable of displaying 2 colours
  • Layer 3,1 – Text mode, 640 w x 256 h pixels, 256 colours total, 80 x 32 cells, each capable of displaying 2 colours
  • Layer 3,2 – Graphics mode, 320 w x 256 h pixels, 256 colours total, 40 x 32 cells each capable of displaying 16 colours
  • Layer 3,3 – Graphics mode, 640 w x 256 h pixels, 256 colours total, 80 x 32 cells, each capable of displaying 16 colours

Layers 2,2 as well as 2,3 together with Layer 3 are not currently available to PRINT and INPUT and therefore won't be discussed in this chapter; they are mentioned here for completeness.