There is great confusion about the QL. The question is: in which computer category does the QL fall? The 32, the 16 or the 8-bit computer!?! It depends on the point of view. For now, we can say that the microprocessor, the MC68008, which is the heart of the QL, is an 8/32-bit type.
The microprocessor or the Central Processing Unit (CPU) is the nerve center of all activities in the computer system. It has generally 3 major tasks:
- Control of operations
- Interpretation of data
- Execution of instruction
But a CPU alone is not very useful. The CPU needs memory and some peripherals to communicate with the outside world. The communication is possible via a computer BUS made up of multiple connectors which carry information to or from the CPU. A computer has generally three main bus types:
- Address bus
- Data bus
- Control bus
Until now, most home computers were built around an 8-bit CPU. (8-bit CPUs are designed to work on 8-bit words at a time.) The two most popular of these were the Z80 (ZX81, TS2068, TANDY) and the 6502 (C64, ATARI). The 68008 of the QL is, like we previously called, an 8/32-bit because it has an 8-bit wide data bus and works internally on 32-bit words. On this point, the 68008 is similar to the INTEL 8088 (IBM PC) which is an 8/16-bit CPU because of its 8-bit data bus and 16-bit internal structure. Apple’s Macintosh is a 16/32-bit with its MC68000.
MC68008 vs MC68000: THE HARDWARE SIDE
The MC68008 is the “economical version” of the MC68000. In fact, the MOTOROLA’s 68000 series forms a real family. The 68000, itself, has a 16-bit wide data bus, the 68008 has an 8-bit one and the 68020 has the 32-bit wide data bus. The “advantage” of the 68008 over the other members of the family is, like we said, economical because the most commonly available low cost support chips are organised on an 8-bit basis. This means that these can be more easily designed into a system around the MC68008. Also the necessary connectors are smaller on 8-bit data bus systems than on a 16-bit data bus systems.
The address bus has been reduced on the 68008 to 20 address lines (24 on the 68000) and 3 of the 68000’s control signals are not directly present on the 68008. These omissions are justified by the fact that the 68000 is a 64-pin I.C. and the 68008 is a 48-pin I.C.. With 20 address lines, the QL can address directly 1MB of memory. There is no need here for a complicated BANK SWITCHING system like on the COMMODORE 128 and ATARI 130XE. On the QL, all the memory is accessible anytime. The 68000 has 2 pins not present on the 68008, UDS (upper data strobe) and LDS (low data strobe), but on the 68008 the pin DS (data strobe) does the same job. Also the VMA (Valid Memory Address) is not present, an external circuit is required here to simulate it.
MC68008 vs MC68000: THE SOFTWARE SIDE
The 68008 has the SAME instruction set as the other members of the 68000 family. With only 56 instruction types and the 14 different addressing modes, the programmer can access up to 1000 powerful instructions. These instructions can be from 1 to 5 words long, that is 2 to 10 bytes long. The 68008 (like the 68000) has 17 all-purpose 32-bit wide registers, one 16-bit FLAGS register and a 32-bit Program Counter. The 32-bit all-purpose registers means SPEED because complex operations can be carried out internally to the 68008 with the minimum need of data to or from memory. Each one of the all-purpose registers can be used like an Accumulator and we can do operations on 1 bit, 4 bits (DIGIT), 8 bits (BYTE), 16 bits (WORD) and 32 bits (LONG WORD).
In conclusion, if you love machine code programming then the Sinclair QL is certainly a good choice.
Editor’s Note: Mr. Gagnon is the editor of SIN, “the only French North American Sinclair oriented magazine”.